Get Unix timestamp (in seconds) for a hg commit - date

Question

Given a commit with the date Mon Aug 18 21:05:38 2014 +0200, how can I get the Unix timestamp of it in seconds?
The following command produces a number that discards the number (presumably because the timezone information of date got discarded):
$ hg log -l1 --template '{date(date, "%s")}\n'
1408392338
$ date -d#1408392338
Mon Aug 18 22:05:38 CEST 2014
I am effectively looking for the equivalent of the git command that produces the commit date as a Unix timestamp:
git log -n1 --pretty=%ct

Answer 1

The simplest solution is to take the first field produced from the template format {date|hgdate}, which is already in UTC. The second field in this format just gives the timezone, and can be discarded by applying Mercurial's word function within the format specifier. Here is a test of this format against the system date command and your own (correct, but longer) answer, on Mercurial 3.7.3:
$ date +%s && hg commit -m 'ok'
1539258496
$ TZ=UTC hg log -l1 --template '{date(date|localdate, "%s")}\n'
1539258496
$ hg log -l1 -r. --template '{date|hgdate}'
1539258496 -7200
$ hg log -l1 --template '{word(0, date|hgdate)}'
1539258496
The correctness is also confirmed by the description in the hg manpage of the hgdate filter:
hgdate Date. Returns the date as a pair of numbers: "1157407993 25200" (Unix timestamp, timezone offset).
(Side note: The timezone offset, a little confusingly, is negated: I'm in UTC+2, but the offset is given as -7200 seconds. However, this is only important if you want the local time. If you're after UTC, you will be ignoring the offset in any case.)
Overall, the date|hgdate variant only saves a few characters, but personally I find it conceptually cleaner and easier to understand. It may also be a little more robust against potential future bugs in Mercurial, since it involves fewer parsing and formatting operations.

Answer 2

The requested timestamp was one that is independent of the timezone, so UTC time. As date(..., "%s") produces a number which is relative to the current timezone, one should request a UTC output by combining the localdate filter with the TZ environment variable to set a timezone:
TZ=UTC hg log -l1 --template '{date(date|localdate, "%s")}\n')

Answer 3

You can get date of changeset in any format, which understand -d option of date and use it in date -d -F
You can use keyword with filter
Sample
hg log -r tip -T "date: {date}\nhgdate: {date|hgdate}\nISO-date: {date|isodate}\n"
date: 1390885140.0-21600
hgdate: 1390885140 -21600
ISO-date: 2014-01-28 10:59 +0600
I think (too lazy to test), result of expression 1390885140-21600 will give you correct timestamp