time: genzabbrs.go skips Feb when checking months

getAbbrs looks like it is checking each month looking for a change
in the time zone abbreviation, but starts in Dec of the previous year
and skips the month of February because of the overflow rules for
AddDate. Changing the day to 1 starts at Jan 1 and tries all months
in the current year. This isn't very important or likely to change
output as zones usually span several months. Discovered when
looking into time.AddDate behavior when adding months.

Change-Id: I685254c8d21c402ba82cc4176e9a86b64ce8f7f7
Reviewed-on: https://go-review.googlesource.com/23322Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/time/genzabbrs.go

@@ -30,7 +30,7 @@ var filename = flag.String("output", "zoneinfo_abbrs_windows.go", "output file n
 // getAbbrs finds timezone abbreviations (standard and daylight saving time)
 // for location l.
 func getAbbrs(l *time.Location) (st, dt string) {
-	t := time.Date(time.Now().Year(), 0, 0, 0, 0, 0, 0, l)
+	t := time.Date(time.Now().Year(), 0, 1, 0, 0, 0, 0, l)
 	abbr1, off1 := t.Zone()
 	for i := 0; i < 12; i++ {
 		t = t.AddDate(0, 1, 0)