mod_archive: Fix hour offset in stored message date
mod_archive: Fix hour offset in stored message date

os.date expect a timestamp in local time, that is subject to daylight saving.
But since we pass an UTC timestamp to os.date one hour is (wrongly) added in
the summer.

The only sensible thing is to call the os.date only once with the ! parametter.
And then parsing this sting to get the utc_timestamp.
Calling os.date with an UTC timestamp is not possible, and calling os.date
twice without timestamp could give different results.

prosody-modules/import2,prosody-modules/import2,prosody-modules/import2,prosody-modules/import2,prosody-modules/import2