I want to acquire the UNIX timestamp value (seconds since January 1st 1970) for an offset from the present, while rounding the result to the beginning of the month.
Essentially I want this output -- two months from current date, but the first day of that month.
date -d "2019-04-01" "+%s"
1554102000
However I don't want to manually add the date. I want to do something similar to this:
date -d +"%Y%m" --date="+2 month" +"%s"
1555973219
but unfortunately this gives me the incorrect timestamp (the result is not rounded to the beginning of the month).
Demonstrating that this all works correctly when using GNU date:
set -x
date
start_of_2mo_hence=$(date -d 'now + 2 months' +%Y-%m-01)
start_of_2mo_hence_sec=$(date -d "$start_of_2mo_hence" +%s)
date -d "@$start_of_2mo_hence_sec"
...emits:
+ date
Fri Feb 22 17:04:49 CST 2019
++ date -d 'now + 2 months' +%Y-%m-01
+ start_of_2mo_hence=2019-04-01
++ date -d 2019-04-01 +%s
+ start_of_2mo_hence_sec=1554094800
+ date -d @1554094800
Mon Apr 1 00:00:00 CDT 2019
Of course, this could be simplified to:
start_of_2mo_hence_sec=$(date -d "$(date -d 'now + 2 months' +%Y-%m-01)" +%s)