Convert custom date format (GMT-08-2021.08.31-18.29.05) to epoch time

Question

I have a date string that is supplied in the following format:

Timezone-Year.Month.Day-Hour.Minute.Second
GMT-08-2021.08.31-18.29.05

and I need to figure out how to get the date command to recognize it so I can convert it to epoch time. This will be done in a bash script in case that matters.

Answer 1

Assuming the timezone is always in the format GMT-nn, something like this could work to shuffle the fields around for the GNU version of date and date -d:

a="GMT-08-2021.08.31-18.29.05"
b=$(echo "$a" | sed -E -e 's/GMT([-+][0-9]+)-([0-9]+)\.([0-9]+)\.([0-9]+)-([0-9]+)\.([0-9]+)\.([0-9]+)/\2-\3-\4 \5:\6:\7 \100/')
date -u -d "$b"

That's basically just a bunch of capture groups ((...)) and putting them back in the right order (\2 for the second group, etc.).

With that example value, b would contain 2021-08-31 18:29:05 -0800, and I get Wed Sep 1 02:29:05 UTC 2021 from date.

For other versions of date, you'd need some other output format.