Parse non standard date format - bash

Question

Say I have the following format outputted for my date:
date --utc +%d.%m.%Y,\ %H:%M\ UTC
# Outputs: 12.06.2014, 09:03 UTC
How can I display the outputted date above, in another date call, in another format? I tried:
date --utc --date="12.06.2014, 09:03 UTC" +%d.%m.%Y,\ %H:%M\ UTC
but with no success (it says invalid date).
I am primarily trying to do this in order to be able to tell from an outputted date how many hours have passed (or days, or whatever time measuring unit).

Answer 1

Here is what the man date page says about format for the --date option:
The --date=STRING is a mostly free format human readable date string such as
"Sun, 29 Feb 2004 16:21:42 -0800" or "2004-02-29 16:21:42" or even "next
Thursday". A date string may contain items indicating calendar date, time of day,
time zone, day of week, relative time, relative date, and numbers. An empty
string indicates the beginning of the day. The date string format is more
complex than is easily documented here but is fully described in the info
documentation.
Hence you can use, for example:
date --date "2014-06-12 09:03 UTC" --utc +%d.%m.%Y,\ %H:%M\ UTC
# Output: 12.06.2014, 09:03 UTC
to get what you desire.
You could get this second form easily from your first output with a sed line as follows:
sed 's/\([0-9]\{2\}\)\.\([0-9]\{2\}\)\.\([0-9]\{4\}\), \(.*\)/\3-\2-\1 \4/'
<<< '12.06.2014, 09:03 UTC'
# Output: 2014-06-12 09:03 UTC
Note that it would probably be faster to output date at ISO 8601 format in the first time for reuse, e.g. with:
date --utc +%F\ %H:%M\ UTC
# Output: 2014-06-12 10:12 UTC

Answer 2

I think you cannot specify input format, so you'll have to change it with another command like this:
date --utc --date="$(echo "12.06.2014, 09:03 UTC" | sed -r 's/(..).(..).(....), (..):(..) UTC/\3-\2-\1 \4:\5 UTC/')"
Also if you want to make arithmethic on this you could use +%s:
DATE1=$(date "+%s" --date="$(echo "12.06.2014, 09:03 UTC" | sed -r 's/(..).(..).(....), (..):(..) UTC/\3-\2-\1 \4:\5 UTC/')")
DATE2=$(date "+%s" --date="$(echo "17.06.2014, 08:30 UTC" | sed -r 's/(..).(..).(....), (..):(..) UTC/\3-\2-\1 \4:\5 UTC/')")
DIFF_IN_SECONDS=$(($DATE2-$DATE1))
DIFF_IN_RAW_DAYS=$(( ($DATE2-$DATE1)/86400 ))
DIFF_IN_DATES=$(( (($DATE2/86400) - ($DATE1/86400)) ))