Merge branch 'jk/strbuf-addftime-seconds-since-epoch'

The "--date=format:<strftime>" gained a workaround for the lack of
system support for a non-local timezone to handle "%s" placeholder.

* jk/strbuf-addftime-seconds-since-epoch:
  strbuf_addftime(): handle "%s" manually

Documentation/rev-list-options.txt

@@ -1047,7 +1047,7 @@ omitted.
 has no effect.
 
 `--date=format:...` feeds the format `...` to your system `strftime`,
-except for %z and %Z, which are handled internally.
+except for %s, %z, and %Z, which are handled internally.
 Use `--date=format:%c` to show the date in your system locale's
 preferred format.  See the `strftime` manual for a complete list of
 format placeholders. When using `-local`, the correct syntax is

cache.h

@@ -1588,6 +1588,7 @@ timestamp_t approxidate_careful(const char *, int *);
 timestamp_t approxidate_relative(const char *date);
 void parse_date_format(const char *format, struct date_mode *mode);
 int date_overflows(timestamp_t date);
+time_t tm_to_time_t(const struct tm *tm);
 
 #define IDENT_STRICT	       1
 #define IDENT_NO_DATE	       2

date.c

@@ -9,7 +9,7 @@
 /*
  * This is like mktime, but without normalization of tm_wday and tm_yday.
  */
-static time_t tm_to_time_t(const struct tm *tm)
+time_t tm_to_time_t(const struct tm *tm)
 {
 	static const int mdays[] = {
 	    0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334

strbuf.c

@@ -1006,7 +1006,12 @@ void strbuf_addftime(struct strbuf *sb, const char *fmt, const struct tm *tm,
 
 	/*
 	 * There is no portable way to pass timezone information to
-	 * strftime, so we handle %z and %Z here.
+	 * strftime, so we handle %z and %Z here. Likewise '%s', because
+	 * going back to an epoch time requires knowing the zone.
+	 *
+	 * Note that tz_offset is in the "[-+]HHMM" decimal form; this is what
+	 * we want for %z, but the computation for %s has to convert to number
+	 * of seconds.
 	 */
 	for (;;) {
 		const char *percent = strchrnul(fmt, '%');
@@ -1019,6 +1024,13 @@ void strbuf_addftime(struct strbuf *sb, const char *fmt, const struct tm *tm,
 			strbuf_addstr(&munged_fmt, "%%");
 			fmt++;
 			break;
+		case 's':
+			strbuf_addf(&munged_fmt, "%"PRItime,
+				    (timestamp_t)tm_to_time_t(tm) -
+				    3600 * (tz_offset / 100) -
+				    60 * (tz_offset % 100));
+			fmt++;
+			break;
 		case 'z':
 			strbuf_addf(&munged_fmt, "%+05d", tz_offset);
 			fmt++;

t/t0006-date.sh

@@ -63,6 +63,10 @@ check_show 'format-local:%%z' "$TIME" '%z'
 check_show 'format:%Y-%m-%d %H:%M:%S' "$TIME" '2016-06-15 16:13:20'
 check_show 'format-local:%Y-%m-%d %H:%M:%S' "$TIME" '2016-06-15 09:13:20' '' EST5
 
+check_show 'format:%s' '123456789 +1234' 123456789
+check_show 'format:%s' '123456789 -1234' 123456789
+check_show 'format-local:%s' '123456789 -1234' 123456789
+
 # arbitrary time absurdly far in the future
 FUTURE="5758122296 -0400"
 check_show iso       "$FUTURE" "2152-06-19 18:24:56 -0400" TIME_IS_64BIT,TIME_T_IS_64BIT