Convert a number of seconds elapsed to date from arbitrary start date

Question

I want to convert a particular amount of seconds to a date. In my case the it's the number of seconds elapsed since 1st of January 0001.

If it were for seconds elapsed from epoch it would be easy: $ date -r nr_of_seconds. It would be awesome if there was a way of telling date to start at a particular date. Is there such an option (the -v option almost does what I need, ... I think)?

I'm on a Mac.

Answer 1

date -r almost does the job. All you need to do is shift the origin, which is an addition.

date -r $((number_of_seconds - epoch))

where epoch is the number of seconds between 1 January 1 and 1 January 1970. The value of epoch depends on your calendar.

In the Gregorian calendar, there are 477 leap years between 1 and 1970, so 365 * 1969 + 477 = 719162 days = 62135596800 seconds. Note that this number is greater than 2³², so you'll need a shell capable of 64-bit arithmetic to handle it. Your number_of_seconds will be more than 2³² anyway if it represents dates beyond the second century AD. I think bash supports 64-bit arithmetic even on older, 32-bit OSX but I'm not sure.

date -r $((number_of_seconds - 62135596800))

Answer 2

You can use this:

echo $(( `date +%s` - ` date --date=yesterday +%s ` ))
86400

You can replace 'yesterday' by any date. For example:

echo $(( `date +%s` - ` date --date='Tue Aug 9 11:44:34 CEST 2014' +%s ` ))
259209

Tested on GNU/Linux.

Answer 3

The problem is that not everyone agrees on what the pre-1923 dates are. We (westerners) are now using the Gregorian calendar. Before 1582, people were using the Julian calendar. In between 1582 and 1923, some people used one or the other.

For instance, England switched to the Gregorian calendar in 1752 and that's what the cal command uses as the switching point. See the output of cal 9 1752.

So if you find an old English document that refers to the 3^rd of November 1654, that's not necessarily the same date as the same 3^rd of November 1654 referred to in a French document. Both the French and English people at the time would agree on what date 0001-01-01 was (as in for instance what happened on that day), but that's not the date GNU date for instance thinks it was.

I once wrote a few POSIX shell functions for arbitrary date calculation using the same calendar switch as cal and ignoring leap seconds. You can find it at: https://github.com/stephane-chazelas/misc-scripts/blob/master/wide_strftime.sh

For instance, to know what the date was 10G seconds (assuming 86400 seconds per day, so not all seconds lasting the same amount of time) after 0001-01-01 00:00:00 GMT Julian Calendar:

$ timegm 1 1 1 0 0 0
$ wide_strftime "%c" "$((REPLY + 10000000000))"
echo "$REPLY"
$ echo "$REPLY"
Mon Nov 18 17:46:40 0317