date.c: skip fractional second part of ISO-8601

git-commit(1) says ISO-8601 is one of our supported date format.

ISO-8601 allows timestamps to have a fractional number of seconds.
We represent time only in terms of whole seconds, so we never bothered
parsing fractional seconds. However, it's better for us to parse and
throw away the fractional part than to refuse to parse the timestamp
at all.

And refusing parsing fractional second part may confuse the parse to
think fractional and timezone as day and month in this example:

	2008-02-14 20:30:45.019-04:00

While doing this, make sure that we only interpret the number after the
second and the dot as fractional when and only when the date is known,
since only ISO-8601 allows the fractional part, and we've taught our
users to interpret "12:34:56.7.days.ago" as a way to specify a time
relative to current time.

Reported-by: Brian M. Carlson <sandals@crustytoothpaste.net>
Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Đoàn Trần Công Danh <congdanhqx@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/date-formats.txt

@@ -20,7 +20,10 @@ RFC 2822::
 ISO 8601::
 	Time and date specified by the ISO 8601 standard, for example
 	`2005-04-07T22:13:13`. The parser accepts a space instead of the
-	`T` character as well.
+	`T` character as well. Fractional parts of a second will be ignored,
+	for example `2005-04-07T22:13:13.019` will be treated as
+	`2005-04-07T22:13:13`
+
 +
 NOTE: In addition, the date part is accepted in the following formats:
 `YYYY.MM.DD`, `MM/DD/YYYY` and `DD.MM.YYYY`.

date.c

@@ -553,6 +553,11 @@ static int set_time(long hour, long minute, long second, struct tm *tm)
 	return -1;
 }
 
+static int is_date_known(struct tm *tm)
+{
+	return tm->tm_year != -1 && tm->tm_mon != -1 && tm->tm_mday != -1;
+}
+
 static int match_multi_number(timestamp_t num, char c, const char *date,
 			      char *end, struct tm *tm, time_t now)
 {
@@ -571,6 +576,13 @@ static int match_multi_number(timestamp_t num, char c, const char *date,
 		if (num3 < 0)
 			num3 = 0;
 		if (set_time(num, num2, num3, tm) == 0) {
+			/*
+			 * If %H:%M:%S was just parsed followed by: .<num4>
+			 * Consider (& discard) it as fractional second
+			 * if %Y%m%d is parsed before.
+			 */
+			if (*end == '.' && isdigit(end[1]) && is_date_known(tm))
+				strtol(end + 1, &end, 10);
 			break;
 		}
 		return 0;

t/t0006-date.sh

@@ -81,6 +81,8 @@ check_parse 2008-02 bad
 check_parse 2008-02-14 bad
 check_parse '2008-02-14 20:30:45' '2008-02-14 20:30:45 +0000'
 check_parse '2008-02-14 20:30:45 -0500' '2008-02-14 20:30:45 -0500'
+check_parse '2008.02.14 20:30:45 -0500' '2008-02-14 20:30:45 -0500'
+check_parse '2008-02-14 20:30:45.019-04:00' '2008-02-14 20:30:45 -0400'
 check_parse '2008-02-14 20:30:45 -0015' '2008-02-14 20:30:45 -0015'
 check_parse '2008-02-14 20:30:45 -5' '2008-02-14 20:30:45 +0000'
 check_parse '2008-02-14 20:30:45 -5:' '2008-02-14 20:30:45 +0000'
@@ -103,6 +105,7 @@ check_approxidate 5.seconds.ago '2009-08-30 19:19:55'
 check_approxidate 10.minutes.ago '2009-08-30 19:10:00'
 check_approxidate yesterday '2009-08-29 19:20:00'
 check_approxidate 3.days.ago '2009-08-27 19:20:00'
+check_approxidate '12:34:56.3.days.ago' '2009-08-27 12:34:56'
 check_approxidate 3.weeks.ago '2009-08-09 19:20:00'
 check_approxidate 3.months.ago '2009-05-30 19:20:00'
 check_approxidate 2.years.3.months.ago '2007-05-30 19:20:00'