Parsing complicated date column

Question

I have a date column that contains dates public opinion polls occur.

These polls occasionally run over several days (usually but not always continuously), the polls sometimes start in one month and finish in the next, and the year has occasionally been entered as YY and other times as YYYY.

Where there is date range it's normally separated using a -, but sometimes – has been used, there are sometimes spaces between dates in a range.

I need to clean this into a consistent date format with a start_date and end_date column. Where polls occur on a single day I'd the end_date columns should be NA or filled with the start date (if you have a solution that does either I can always work from there to do the reverse if needed). Where there are non-continuous date ranges, the earliest date and latest date and the intermediate stop and restarts can be discarded.

Because the formatting is so annoyingly inconsistent I've provided the full the data since any solution would need to work on all dates in the date set (or work on some of them and not break the others so we could solve the issue iteratively).

dates <- c("12-15 Feb 2019", "6–11 Feb 2019", "7–10 Feb 2019", "23–30 Jan 2019", 
"24–27 Jan 2019", "9–13 Jan 2019", "13-16 Dec 2018", "13–15 Dec 2018", 
"6–9 Dec 2018", "29 Nov – 2 Dec 2018", "23–25 Nov 2018", "15-18 Nov 2018", 
"15–17 Nov 2018", "8–11 Nov 2018", "1–4 Nov 2018", "25–28 Oct 2018", 
"19–21 Oct 2018", "10–13 Oct 2018", "10–13 Oct 2018", "5–7 Oct 2018", 
"22–24 Sep 2018", "20–23 Sep 2018", "12–15 Sep 2018", "8–10 Sep 2018", 
"6–9 Sep 2018", "25–26 Aug 2018", "24–26 Aug 2018", "24–25 Aug 2018", 
"15-18 Aug 2018", "12-Aug-18", "06-Aug-18", "29-Jul-18", "17-Jul-18", 
"16-Jul-18", "03-Jul-18", "02-Jul-18", "21–24 Jun 2018", "14–17 Jun 2018", 
"14–17 Jun 2018", "02-Jun-18", "31 May – 3 Jun 2018", "24–27 May 2018", 
"17–20 May 2018", "10–13 May 2018", "10–13 May 2018", "10–12 May 2018", 
"3–6 May 2018", "30-Apr-18", "19–22 Apr 2018", "22-Apr-18", "5–8 Apr 2018", 
"5–8 Apr 2018", "3–5 Apr 2018", "24 Mar – 1 Apr 2018", "28-Mar-18", 
"22–25 Mar 2018", "22–25 Mar 2018", "17–25 Mar 2018", "8–11 Mar 2018", 
"3–11 Mar 2018", "1–4 Mar 2018", "22–25 Feb 2018", "24-Feb-18", 
"15–18 Feb 2018", "8–11 Feb 2018", "1–3 Feb 2018", "26–28 Jan 2018", 
"25-Jan-18", "11–15 Jan 2018", "19-Dec-17", "14–17 Dec 2017", 
"12-Dec-17", "7–10 Dec 2017", "05-Dec-17", "30 Nov ? 3 Dec 2017", 
"29-Nov-17", "28-Nov-17", "23–27 Nov 2017", "21-Nov-17", "14-Nov-17", 
"14-Nov-17", "13-Nov-17", "30-Oct-17", "26–29 Oct 2017", "24-Oct-17", 
"12–15 Oct 2017", "04-Oct-17", "01-Oct-17", "26-Sep-17", "21–24 Sep 2017", 
"19-Sep-17", "14–18 Sep 2017", "12-Sep-17", "6–9 Sep 2017", "05-Sep-17", 
"31 Aug – 4 Sep 2017", "28 Aug – 2 Sep 2017", "29-Aug-17", "23-Aug-17", 
"22-Aug-17", "17–21 Aug 2017", "17–20 Aug 2017", "15-Aug-17", 
"08-Aug-17", "3–6 Aug 2017", "01-Aug-17", "25-Jul-17", "20–24 Jul 2017", 
"20–23 Jul 2017", "19-Jul-17", "18-Jul-17", "6–11 Jul 2017", 
"6–9 Jul 2017", "29-Jun-17", "22–27 Jun 2017", "15–18 Jun 2017", 
"14-Jun-17", "26–29 May 2017", "23-May-17", "12–15 May 2017", 
"11-May-17", "10–11 May 2017", "26–30 Apr 2017", "20–23 Apr 2017", 
"13–16 Apr 2017", "6–9 Apr 2017", "1–4 Apr 2017", "30 Mar – 2 Apr 2017", 
"24–27 Mar 2017", "22–25 Mar 2017", "17–20 Mar 2017", "16–19 Mar 2017", 
"10–13 Mar 2017", "3–6 Mar 2017", "23–26 Feb 2017", "16–19 Feb 2017", 
"9–12 Feb 2017", "2–5 Feb 2017", "20–23 Jan 2017", "13–16 Jan 2017", 
"12-Jan-17", "9–12 Dec 2016", "1–4 Dec 2016", "25–28 Nov 2016", 
"24–26 Nov 2016", "17–20 Nov 2016", "11–14 Nov 2016", "3–6 Nov 2016", 
"20–23 Oct 2016", "14–17 Oct 2016", "7–10 Oct 2016", "6–9 Oct 2016", 
"22–25 Sep 2016", "9–12 Sep 2016", "8–11 Sep 2016", "26–29 Aug 2016", 
"25–28 Aug 2016", "19–22 Aug 2016", "12–15 Aug 2016", "5–8 Aug 2016", 
"27 Jul – 1 Aug 2016", "20–24 Jul 2016", "13–17 Jul 2016", "6–10 Jul 2016", 
"30 Jun – 3 Jul 2016", "28 Jun – 1 Jul 2016", "30-Jun-16", "27–30 Jun 2016", 
"28–29 Jun 2016", "26–29 Jun 2016", "28 Jun – 1 Jul 2016", "30-Jun-16", 
"27–30 Jun 2016", "28–29 Jun 2016", "26–29 Jun 2016", "23–26 Jun 2016", 
"23–26 Jun 2016", "23-Jun-16", "20–22 Jun 2016", "16–19 Jun 2016", 
"16–19 Jun 2016", "16-Jun-16", "14–16 Jun 2016", "9–12 Jun 2016", 
"09-Jun-16", "2–5 Jun 2016", "2–5 Jun 2016", "02-Jun-16", "31 May – 2 Jun 2016", 
"26–29 May 2016", "21–22,\n                      28–29 May 2016", 
"26-May-16", "19–22 May 2016", "19–22 May 2016", "19-May-16", 
"17–19 May 2016", "14–15 May 2016", "12–15 May 2016", "6–8 May 2016", 
"5–8 May 2016", "5–8 May 2016", "5–7 May 2016", "4–6 May 2016", 
"05-May-16", "27 Apr – 1 May 2016", "23–24, 30 Apr – 1 May 2016", 
"20–24 Apr 2016", "14–17 Apr 2016", "13–17 Apr 2016", "9–10,\n                      16–17 Apr 2016", 
"14–16 Apr 2016", "14-Apr-16", "6–10 Apr 2016", "31 Mar – 3 Apr 2016", 
"26–27 Mar, 2–3 Apr 2016", "21-Mar-16", "17–20 Mar 2016", "16–20 Mar 2016", 
"12–13,\n                      19–20 Mar 2016", "10–12 Mar 2016", 
"3–6 Mar 2016", "2–6 Mar 2016", "27–28 Feb, 5–6 Mar 2016", "24–28 Feb 2016", 
"18–21 Feb 2016", "17–21 Feb 2016", "13–14, 20–21 Feb 2016", 
"11–13 Feb 2016", "11-Feb-16", "3–7 Feb 2016", "30–31 Jan,\n                      6–7 Feb 2016", 
"28–31 Jan 2016", "16–17, 23–24 Jan 2016", "21-Jan-16", "15–18 Jan 2016", 
"2–3, 9–10 Jan 2016", "15-Dec-15", "5–6, 12–13 Dec 2015", "08-Dec-15", 
"4–6 Dec 2015", "01-Dec-15", "21–22, 28–29 Nov 2015", "26-Nov-15", 
"24-Nov-15", "19–22 Nov 2015", "7–8, 14–15 Nov 2015", "12–14 Nov 2015", 
"10-Nov-15", "6–8 Nov 2015", "03-Nov-15", "24–25 Oct,\n                      1 Nov 2015", 
"27-Oct-15", "23–25 Oct 2015", "22-Oct-15", "20-Oct-15", "10–11, 17–18 Oct 2015", 
"15–17 Oct 2015", "13-Oct-15", "9–11 Oct 2015", "26–27 Sep, 1–5 Oct 2015", 
"1–4 Oct 2015", "24–28 Sep 2015", "17–21 Sep 2015", "19–20 Sep 2015", 
"17–20 Sep 2015", "15–16 Sep 2015", "15-Sep-15", "12–13 Sep 2015", 
"5–6 Sep 2015", "4–6 Sep 2015", "26–30 Aug 2015", "27-Aug-15", 
"22–23 Aug 2015", "20–23 Aug 2015", "13–15 Aug 2015", "11–14 Aug 2015", 
"8–9 Aug 2015", "8–9 Aug 2015", "4–7 Aug 2015", "06-Aug-15", 
"28–31 Jul 2015", "30-Jul-15", "25–26 Jul 2015", "16–19 Jul 2015", 
"14–17 Jul 2015", "11–12 Jul 2015", "4–5 Jul 2015", "2–4 Jul 2015", 
"27–28 Jun 2015", "16-Jun-15", "16-Jun-15", "13–14 Jun 2015", 
"11–13 Jun 2015", "11–13 Jun 2015", "02-Jun-15", "02-Jun-15", 
"23–24, 30–31 May 2015", "26-May-15", "18-May-15", "17-May-15", 
"17-May-15", "13-May-15", "7–10 May 2015", "04-May-15", "04-May-15", 
"28-Apr-15", "21-Apr-15", "11–12,\n                      18–19 Apr 2015", 
"14-Apr-15", "10–12 Apr 2015", "9–11 Apr 2015", "28–29 Mar, 3–6 Apr 2015", 
"29-Mar-15", "20–22 Mar 2015", "14–15, 21–22 Mar 2015", "17-Mar-15", 
"10-Mar-15", "7–8 Mar 2015", "28 Feb–1, 7–8 Mar 2015", "26–28 Feb 2015", 
"20–22 Feb 2015", "20–22 Feb 2015", "6–8 Feb 2015", "31 Jan–1, 7–8 Feb 2015", 
"05-Feb-15", "4–5 Feb 2015", "28–30 Jan 2015", "27-Jan-15", "r27 Jan 2015", 
"20-Jan-15", "13-Jan-15", "12-Jan-15", "23–27 Dec 2014", "16-Dec-14", 
"12–15 Dec 2014", "6–7, 13–14 Dec 2014", "4–6 Dec 2014", "2–4 Dec 2014", 
"02-Dec-14", "29–30 Nov 2014", "22–23, 29–30 Nov 2014", "25-Nov-14", 
"21-Nov-14", "18-Nov-14", "17-Nov-14", "17-Nov-14", "11-Nov-14", 
"04-Nov-14", "04-Nov-14", "25–26 Oct,\n                      1–2 Nov 2014", 
"30 Oct–1 Nov 2014", "28-Oct-14", "23-Oct-14", "21-Oct-14", "21-Oct-14", 
"20-Oct-14", "14-Oct-14", "07-Oct-14", "4–5 Oct 2014", "4–5 Oct 2014", 
"23-Sep-14", "13–14,\n                      20–21 Sep 2014", 
"18-Sep-14", "30–31 Aug, 6–7 Sep 2014", "5–7 Sep 2014", "22–24 Aug 2014", 
"16–17, 23–24 Aug 2014", "19-Aug-14", "9–10 Aug 2014", "8–10 Aug 2014", 
"25–27 Jul 2014", "11–13 Jul 2014", "01-Jul-14", "30-Jun-14", 
"27–29 Jun 2014", "13–15 Jun 2014", "30 May–1 Jun 2014", "27-May-14", 
"20-May-14", "17–18 May 2014", "16–18 May 2014", "15–17 May 2014", 
"04-May-14", "2–4 May 2014", "30-Apr-14", "22-Apr-14", "15-Apr-14", 
"13-Apr-14", "08-Apr-14", "07-Apr-14", "4–6 Apr 2014", "25-Mar-14", 
"25-Mar-14", "21–23 Mar 2014", "18-Mar-14", "13–15 Mar 2014", 
"7–9 Mar 2014", "05-Mar-14", "23-Feb-14", "21–23 Feb 2014", "15-Feb-14", 
"7–9 Feb 2014", "28-Jan-14", "23-Jan-14", "17–20 Jan 2014", "13-Jan-14", 
"16-Dec-13", "15-Dec-13", "6–8 Dec 2013", "28 Nov–2 Dec 2013", 
"30 Nov–1 Dec 2013", "22–24 Nov 2013", "21–23 Nov 2013", "8–10 Nov 2013", 
"25–27 Oct 2013", "19–20 Oct 2013", "21–22 Sep 2013", "19–22 Sep 2013", 
"12–15 Sep 2013", "4–6 Sep 2013", "05-Sep-13", "3–5 Sep 2013", 
"4–6 Sep 2013", "05-Sep-13", "4–5 Sep 2013", "3–5 Sep 2013", 
"04-Sep-13", "2–4 Sep 2013", "1–4 Sep 2013", "03-Sep-13", "30 Aug–1 Sep 2013", 
"30 Aug–1 Sep 2013", "29 Aug–1 Sep 2013", "28–29 Aug 2013", "28–29 Aug 2013", 
"26-Aug-13", "21–25 Aug 2013", "23–25 Aug 2013", "23–25 Aug 2013", 
"18–22 Aug 2013", "16–18 Aug 2013", "16–18 Aug 2013", "16–18 Aug 2013", 
"14–18 Aug 2013", "14–15 Aug 2013", "12–13 Aug 2013", "9–12 Aug 2013", 
"9–11 Aug 2013", "9–11 Aug 2013", "10-Aug-13", "7–9 Aug 2013", 
"6–8 Aug 2013", "04-Aug-13", "2–4 Aug 2013", "2–4 Aug 2013", 
"1–4 Aug 2013", "26–28 Jul 2013", "25–28 Jul 2013", "23–25 Jul 2013", 
"18–22 Jul 2013", "19–21 Jul 2013", "19–21 Jul 2013", "18-Jul-13", 
"12–14 Jul 2013", "11–14 Jul 2013", "11–13 Jul 2013", "5–8 Jul 2013", 
"5–7 Jul 2013", "5–7 Jul 2013", "4–7 Jul 2013", "28–30 Jun 2013", 
"28–30 Jun 2013", "27–30 Jun 2013", "27–28 Jun 2013", "27-Jun-13", 
"21–23 Jun 2013", "21–23 Jun 2013", "20–23 Jun 2013", "14–16 Jun 2013", 
"13–16 Jun 2013", "13–15 Jun 2013", "11–13 Jun 2013", "7–10 Jun 2013", 
"6–10 Jun 2013", "31 May–2 Jun 2013", "31 May–2 Jun 2013", "30 May–2 Jun 2013", 
"24–26 May 2013", "23–26 May 2013", "17–19 May 2013", "17–19 May 2013", 
"16–19 May 2013", "16–18 May 2013", "15–16 May 2013", "10–12 May 2013", 
"9–12 May 2013", "3–5 May 2013", "3–5 May 2013", "2–5 May 2013", 
"02-May-13", "26–28 Apr 2013", "25–28 Apr 2013", "18–22 Apr 2013", 
"18–22 Apr 2013", "19–21 Apr 2013", "11–14 Apr 2013", "11–14 Apr 2013", 
"11–13 Apr 2013", "9–11 Apr 2013", "02-May-13", "5–7 Apr 2013", 
"4–7 Apr 2013", "4–7 Apr 2013", "29 Mar–1 Apr 2013", "28 Mar–1 Apr 2013", 
"22–24 Mar 2013", "21–24 Mar 2013", "22–23 Mar 2013", "21–24 Mar 2013", 
"22–25 Mar 2013", "14–17 Mar 2013", "14–17 Mar 2013", "14–16 Mar 2013", 
"7–10 Mar 2013", "7–10 Mar 2013", "8–10 Mar 2013", "5–7 Mar 2013", 
"28 Feb–3 Mar 2013", "28 Feb–3 Mar 2013", "21–24 Feb 2013", "16–17/23–24 Feb 2013", 
"22–24 Feb 2013", "14–17 Feb 2013", "14–16 Feb 2013", "7–10 Feb 2013", 
"9–10 Feb 2013", "1–4 Feb 2013", "2–3 Feb 2013", "1–3 Feb 2013", 
"1–3 Feb 2013", "23–28 Jan 2013", "19–20/26–27 Jan 2013", "16–20 Jan 2013", 
"9–13 Jan 2013", "11–13 Jan 2013", "5–6/12–13 Jan 2013", "12–16 Dec 2012", 
"8–9/15–16 Dec 2012", "13–15 Dec 2012", "5–9 Dec 2012", "7–9 Dec 2012", 
"28 Nov–2 Dec 2012", "24–25 Nov/1–2 Dec 2012", "29–30 Nov 2012", 
"27–29 Nov 2012", "23–25 Nov 2012", "21–25 Nov 2012", "14–18 Nov 2012", 
"10–11/17–18 Nov 2012", "15–17 Nov 2012", "9–11 Nov 2012", "7–11 Nov 2012", 
"2–6 Nov 2012", "2–4 Nov 2012", "27–28 Oct/3–4 Nov 2012", "26–28 Oct 2012", 
"25–28 Oct 2012", "13–14/20–21 Oct 2012", "17–21 Oct 2012", "18–20 Oct 2012", 
"10–14 Oct 2012", "5–7 Oct 2012", "3–7 Oct 2012", "29–30 Sep/6–7 Oct 2012", 
"26–30 Sep 2012", "22–23 Sep 2012", "19–23 Sep 2012", "17–20 Sep 2012", 
"14–16 Sep 2012", "12–16 Sep 2012", "8–9/15–16 Sep 2012", "13–15 Sep 2012", 
"29 Aug–2 Sep 2012", "31 Aug–2 Sep 2012", "1–2 Sep 2012", "22–26 Aug 2012", 
"23–25 Aug 2012", "15–19 Aug 2012", "17–19 Aug 2012", "11–12/18–19 Aug 2012", 
"8–12 Aug 2012", "3–5 Aug 2012", "1–5 Aug 2012", "28–29 Jul/4–5 Aug 2012", 
"25–29 Jul 2012", "26–28 Jul 2012", "20–22 Jul 2012", "18–22 Jul 2012", 
"14–15/21–22 Jul 2012", "11–15 Jul 2012", "6–8 Jul 2012", "4–8 Jul 2012", 
"30 Jun–1/7–8 Jul 2012", "27 Jun–1 Jul 2012", "22–24 Jun 2012", 
"20–24 Jun 2012", "16–17/23–24 Jun 2012", "13–17 Jun 2012", "15–17 Jun 2012", 
"6–11 Jun 2012", "9–10 Jun 2012", "7–10 Jun 2012", "2–3 Jun 2012", 
"31 May–2 Jun 2012", "30 May–3 Jun 2012", "26–27 May 2012", "23–27 May 2012", 
"25–27 May 2012", "16–20 May 2012", "19–20 May 2012", "12–13 May 2012", 
"11–13 May 2012", "9–13 May 2012", "9–10 May 2012", "9–10 May 2012", 
"5–6 May 2012", "2–6 May 2012", "27–29 Apr 2012", "27–29 Apr 2012", 
"25–29 Apr 2012", "21–22 Apr 2012", "18–22 Apr 2012", "17–19 Apr 2012", 
"13–15 Apr 2012", "11–15 Apr 2012", "7–8/14–15 Apr 2012", "4–9 Apr 2012", 
"31 Mar–1 Apr 2012", "28 Mar–1 Apr 2012", "29–31 Mar 2012", "21–25 Mar 2012", 
"24–25 Mar 2012", "23–25 Mar 2012", "14–18 Mar 2012", "10–11/17–18 Mar 2012", 
"9–11 Mar 2012", "7–11 Mar 2012", "3–4 Mar 2012", "29 Feb–4 Mar 2012", 
"25–26 Feb 2012", "23–26 Feb 2012", "22–26 Feb 2012", "23–24 Feb 2012", 
"22–23 Feb 2012", "15–19 Feb 2012", "11–12/18–19 Feb 2012", "10–12 Feb 2012", 
"8–10 Feb 2012", "7–8 Feb 2012", "4–5 Feb 2012", "1–5 Feb 2012", 
"2–4 Feb 2012", "28–29 Jan 2012", "27–29 Jan 2012", "25–29 Jan 2012", 
"27–28 Jan 2012", "18–22 Jan 2012", "14–15/21–22 Jan 2012", "17–18 Jan 2012", 
"11–15 Jan 2012", "7–8 Jan 2012", "14–18 Dec 2011", "10–11/17–18 Dec 2011", 
"7–11 Dec 2011", "8–10 Dec 2011", "2–4 Dec 2011", "30 Nov–4 Dec 2011", 
"26–27 Nov/3–4 Dec 2011", "23–27 Nov 2011", "19–20 Nov 2011", 
"18–20 Nov 2011", "16–20 Nov 2011", "9–13 Nov 2011", "5–6/12–13 Nov 2011", 
"10–12 Nov 2011", "3–6 Nov 2011", "2–6 Nov 2011", "2–3 Nov 2011", 
"26–30 Oct 2011", "29–30 Oct 2011", "25–26 Oct 2011", "22–23 Oct 2011", 
"21–23 Oct 2011", "19–23 Oct 2011", "15–16Oct 2011", "14–16 Oct 2011", 
"12–16 Oct 2011", "13–15 Oct 2011", "8–9 Oct 2011", "7–9 Oct 2011", 
"4–9 Oct 2011", "27 Sep–2 Oct 2011", "24–25 Sep/1–2 Oct 2011", 
"20–25 Sep 2011", "16–18 Sep 2011", "13–18 Sep 2011", "10–11/17–18 Sep 2011", 
"7–11 Sep 2011", "8–10 Sep 2011", "2–4 Sep 2011", "31 Aug–4 Sep 2011", 
"27–28 Aug/3–4 Sep 2011", "24–28 Aug 2011", "19–21 Aug 2011", 
"17–21 Aug 2011", "13–14/20–21 Aug 2011", "10–14 Aug 2011", "11–13 Aug 2011", 
"9–10 Aug 2011", "5–7 Aug 2011", "3–7 Aug 2011", "30–31 Jul/6–7 Aug 2011", 
"c. 3 Aug 2011", "27–31 Jul 2011", "22–24 Jul 2011", "20–24 Jul 2011", 
"16–17/23–24 Jul 2011", "13–17 Jul 2011", "14–16 Jul 2011", "13–14 Jul 2011", 
"9–10 Jul 2011", "8–10 Jul 2011", "6–10 Jul 2011", "29 Jun–3 Jul 2011", 
"25–26 Jun/1–2 Jul 2011", "24–26 Jun 2011", "22–26 Jun 2011", 
"11–12/18–19 Jun 2011", "15–19 Jun 2011", "14–16 Jun 2011", "8–13 Jun 2011", 
"10–12 Jun 2011", "4–5 Jun 2011", "1–5 Jun 2011", "31 May–2 Jun 2011", 
"25–29 May 2011", "27–29 May 2011", "21–22/28–29 May 2011", "18–22 May 2011", 
"14–15 May 2011", "13–15 May 2011", "11–15 May 2011", "12–14 May 2011", 
"7–8 May 2011", "4–8 May 2011", "3–4 May 2011", "29 Apr–1 May 2011", 
"28 Apr–1 May 2011", "23–24/30 Apr–1 May 2011", "20–26 Apr 2011", 
"13–17 Apr 2011", "9–10/16–17 Apr 2011", "14–16 Apr 2011", "6–10 Apr 2011", 
"2–3 Apr 2011", "1–3 Apr 2011", "30 Mar–3 Apr 2011", "26–27 Mar 2011", 
"23–27 Mar 2011", "22–24 Mar 2011", "19–20 Mar 2011", "18–20 Mar 2011", 
"16–20 Mar 2011", "16–17 Mar 2011", "12–13 Mar 2011", "9–13 Mar 2011", 
"10–12 Mar 2011", "8–10 Mar 2011", "5–6 Mar 2011", "4–6 Mar 2011", 
"2–6 Mar 2011", "26–27 Feb 2011", "22–27 Feb 2011", "21–23 Feb 2011", 
"18–20 Feb 2011", "15–20 Feb 2011", "12–13/19–20 Feb 2011", "8–13 Feb 2011", 
"10–12 Feb 2011", "4–6 Feb 2011", "1–6 Feb 2011", "29–30 Jan/5–6 Feb 2011", 
"1–3 Feb 2011", "25–30 Jan 2011", "18–23 Jan 2011", "15–16/22–23 Jan 2011", 
"11–16 Jan 2011", "8–9 Jan 2011", "14–19 Dec 2010", "11–12 Dec 2010", 
"8–12 Dec 2010", "7–12 Dec 2010", "4–5 Dec 2010", "3–5 Dec 2010", 
"30 Nov–5 Dec 2010", "23–28 Nov 2010", "20–21/27–28 Nov 2010", 
"19–21 Nov 2010", "16–21 Nov 2010", "18–20 Nov 2010", "9–14 Nov 2010", 
"6–7/13–14 Nov 2010", "5–7 Nov 2010", "2–7 Nov 2010", "26–31 Oct 2010", 
"23–24/30–31 Oct 2010", "22–24 Oct 2010", "19–24 Oct 2010", "21–23 Oct 2010", 
"12–17 Oct 2010", "9–10/16–17 Oct 2010", "8–10 Oct 2010", "5–10 Oct 2010", 
"2–3 Oct 2010", "30 Sep–1 Oct 2010", "21–26 Sep 2010", "18–19 Sep 2010", 
"14–19 Sep 2010", "15–16 Sep 2010", "10–12 Sep 2010", "7–12 Sep 2010", 
"31 Aug–5 Sep 2010", "28–29 Aug/4–5 Sep 2010", "24–29 Aug 2010", 
"25–26 Aug 2010", "c. 21 Aug 2010", "17–19 Aug 2010", "13–19 Aug 2010"
)

Some particularly odd dates to look out for in the dataset "30 Nov ? 3 Dec 2017"
"21–22,\n 28–29 May 2016" "24–25 Oct,\n 1 Nov 2015" "29–30 Sep/6–7 Oct 2012"

Answer 1

This was an interesting problem! But I think it can be solved with regex.

How about this:

library(tidyverse)
tibble(dates = dates) %>%
  mutate(end_year = str_extract(dates, "[0-9]*$"),
         end_year = ifelse(str_length(end_year) == 2, paste0("20", end_year), end_year),
         month_one = str_extract(dates, "[A-Z][a-z][a-z]"),
         month_two = str_sub(str_extract(dates, "[A-Z][a-z][a-z].*[A-Z][a-z][a-z]"), start = -3),
         month_two = if_else(is.na(month_two), month_one, month_two),
         day_one = str_extract(dates, "[0-9]+"),
         dates_without_day_one = gsub("^[0-9]+", "", dates),
         day_two = str_extract(dates_without_day_one, "[0-9]+"),
         day_two = str_squish(gsub("[-–]", "", day_two)),
         day_three_four = str_extract(dates, "/.+[-–] *[0-9]+"),
         day_three = str_extract(day_three_four, "/ *[0-9]+"),
         day_three = str_squish(gsub("/", "", day_three)),
         day_four = str_extract(day_three_four, "[-–] *[0-9]+"),
         day_four = str_squish(gsub("[-–]", "", day_four))
  ) %>%
  # dates that are only a single day:
  mutate(day_two = if_else(is.na(day_two), day_one, day_two)) %>%
  # dates that actually have four days:
  mutate(day_one = ifelse(is.na(day_three),
                           day_one,
                           round((as.numeric(day_one) + as.numeric(day_two)) / 2)),
         day_two = ifelse(is.na(day_three),
                           day_two,
                           round((as.numeric(day_three) + as.numeric(day_four)) / 2))) %>%
  select(-day_three_four, -dates_without_day_one) %>%

  mutate(start_date = as.Date(paste(end_year,month_one, day_one, sep = "-"), format = "%Y-%b-%d"),
         end_date = as.Date(paste(end_year,month_two, day_two, sep = "-"), format = "%Y-%b-%d")) %>%
  select(dates, start_date, end_date, everything()) 

Delivers:

# A tibble: 838 x 10
   dates               start_date end_date   end_year month_one month_two day_one day_two day_three day_four
   <chr>               <date>     <date>     <chr>    <chr>     <chr>     <chr>   <chr>   <chr>     <chr>   
 1 12-15 Feb 2019      2019-02-12 2019-02-15 2019     Feb       Feb       12      15      NA        NA      
 2 6–11 Feb 2019       2019-02-06 2019-02-11 2019     Feb       Feb       6       11      NA        NA      
 3 7–10 Feb 2019       2019-02-07 2019-02-10 2019     Feb       Feb       7       10      NA        NA      
 4 23–30 Jan 2019      2019-01-23 2019-01-30 2019     Jan       Jan       23      30      NA        NA      
 5 24–27 Jan 2019      2019-01-24 2019-01-27 2019     Jan       Jan       24      27      NA        NA      
 6 9–13 Jan 2019       2019-01-09 2019-01-13 2019     Jan       Jan       9       13      NA        NA      
 7 13-16 Dec 2018      2018-12-13 2018-12-16 2018     Dec       Dec       13      16      NA        NA      
 8 13–15 Dec 2018      2018-12-13 2018-12-15 2018     Dec       Dec       13      15      NA        NA      
 9 6–9 Dec 2018        2018-12-06 2018-12-09 2018     Dec       Dec       6       9       NA        NA      
10 29 Nov – 2 Dec 2018 2018-11-29 2018-12-02 2018     Nov       Dec       29      2       NA        NA