[PATCH v2] date: make "iso-strict" conforming for the UTC timezone

["Beat Bolli" <bb@drbeat.li>]

ISO 8601-1:2020-12 specifies that a zero timezone offset must be denoted
with a "Z" suffix instead of the numeric "+00:00". Add the correponding
special case to show_date() and a new test.

This changes an established output format which might be depended on by
scripts. The original patch 466fb6742d7f (pretty: provide a strict ISO
8601 date format, 2014-08-29) mentioned XML parsers as its rationale,
which generally have good parsing support, so this change should be
fine.

Reported-by: Michael Osipov <michael.osipov@innomotics.com>
Signed-off-by: Beat Bolli <dev+git@drbeat.li>
---
Changes from v1:

- added a comment why the change is fine
- removed the Link: trailer

 date.c          | 14 +++++++++-----
 t/t0006-date.sh |  1 +
 2 files changed, 10 insertions(+), 5 deletions(-)

diff --git a/date.c b/date.c
index 619ada5b2044..44cf2221d81f 100644
--- a/date.c
+++ b/date.c
@@ -342,14 +342,18 @@ const char *show_date(timestamp_t time, int tz, const struct date_mode *mode)
 				tm->tm_hour, tm->tm_min, tm->tm_sec,
 				tz);
 	else if (mode->type == DATE_ISO8601_STRICT) {
-		char sign = (tz >= 0) ? '+' : '-';
-		tz = abs(tz);
-		strbuf_addf(&timebuf, "%04d-%02d-%02dT%02d:%02d:%02d%c%02d:%02d",
+		strbuf_addf(&timebuf, "%04d-%02d-%02dT%02d:%02d:%02d",
 				tm->tm_year + 1900,
 				tm->tm_mon + 1,
 				tm->tm_mday,
-				tm->tm_hour, tm->tm_min, tm->tm_sec,
-				sign, tz / 100, tz % 100);
+				tm->tm_hour, tm->tm_min, tm->tm_sec);
+		if (tz == 0) {
+			strbuf_addch(&timebuf, 'Z');
+		} else {
+			strbuf_addch(&timebuf, tz >= 0 ? '+' : '-');
+			tz = abs(tz);
+			strbuf_addf(&timebuf, "%02d:%02d", tz / 100, tz % 100);
+		}
 	} else if (mode->type == DATE_RFC2822)
 		strbuf_addf(&timebuf, "%.3s, %d %.3s %d %02d:%02d:%02d %+05d",
 			weekday_names[tm->tm_wday], tm->tm_mday,
diff --git a/t/t0006-date.sh b/t/t0006-date.sh
index e18b1602864e..1d228a981ee9 100755
--- a/t/t0006-date.sh
+++ b/t/t0006-date.sh
@@ -46,6 +46,7 @@ check_show () {
 TIME='1466000000 +0200'
 check_show iso8601 "$TIME" '2016-06-15 16:13:20 +0200'
 check_show iso8601-strict "$TIME" '2016-06-15T16:13:20+02:00'
+check_show iso8601-strict "$(echo "$TIME" | sed 's/+0200$/+0000/')" '2016-06-15T14:13:20Z'
 check_show rfc2822 "$TIME" 'Wed, 15 Jun 2016 16:13:20 +0200'
 check_show short "$TIME" '2016-06-15'
 check_show default "$TIME" 'Wed Jun 15 16:13:20 2016 +0200'
-- 
2.44.0