Converting days since epoch to date

Question

How can one convert a serial date number, representing the number of days since epoch (1970), to the corresponding date string? I have seen multiple posts showing how to go from string to date number, but I haven't been able to find any posts on how to do the reverse.

For example, 15951 corresponds to "2013-09-02".

>>> import datetime
>>> (datetime.datetime(2013, 9, 2) - datetime.datetime(1970,1,1)).days + 1
15951

(The + 1 because whatever generated these date numbers followed the convention that Jan 1, 1970 = 1.)

TL;DR: Looking for something to do the following:

>>> serial_date_to_string(15951)  # arg is number of days since 1970
"2013-09-02"

This is different from Python: Converting Epoch time into the datetime because I am starting with days since 1970. I not sure if you can just multiply by 86,400 due to leap seconds, etc.

Answer 1

Use the datetime package as follows:

import datetime
def serial_date_to_string(srl_no):
    new_date = datetime.datetime(1970,1,1,0,0) + datetime.timedelta(srl_no - 1)
    return new_date.strftime("%Y-%m-%d")

This is a function which returns the string as required.

So:

serial_date_to_string(15951)

Returns

>> "2013-09-02"

Answer 2

And for a Pandas Dataframe:

df["date"] = pd.to_datetime(df["date"], unit="d")

... assuming that the "date" column contains values like 18687 which is days from Unix Epoch of 1970-01-01 to 2021-03-01.

Also handles seconds and milliseconds since Unix Epoch, use unit="s" and unit="ms" respectively.

Also see my other answer with the exact reverse.