Convert number of days since Jan 1 2000 into date format

Question

I have a column in my data as a number of days since Jan 1, 2000 (this is 0 day). How can I convert day number into date format (2000-01-01)?

Number_of_days    Date
3536              2011-01-03
3537              2011-01-04
3538              2011-01-05
3539              2011-01-06
3540              2011-01-07
3541              2011-01-08

I want to add a new column called Date similar to above one into my df. Number of days to Date above is just an example.

Answer 1

You need the origin-parameter of as.Date. Using:

as.Date(3536:3541, origin = '2000-01-01')

gives:

[1] "2009-09-06" "2009-09-07" "2009-09-08" "2009-09-09" "2009-09-10" "2009-09-11"

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In your case you can use:

as.Date(df$Number_of_days - 1, origin = '2000-01-01')

or:

as.Date(df$Number_of_days, origin = '1999-12-31')