Here's a reproducer that does not require the machine to be in Brasil

package main

import (
	"fmt"
	"time"
)

func main() {
	br, _ := time.LoadLocation("America/Sao_Paulo")
	t1 := time.Now()
	t1 = time.Date(t1.Year(), t1.Month(), t1.Day(),
		t1.Hour(), t1.Minute(), t1.Second(), 0, br)
	_, err := time.Parse(time.RFC1123, t1.Format(time.RFC1123))
	if err != nil {
		fmt.Println(err)
	}
}

$ go run prova.go
parsing time "Tue, 01 Aug 2017 17:35:45 -03" as "Mon, 02 Jan 2006 15:04:05 MST": cannot parse "-03" as "MST"

The issue is in the time.Parse(time.RFC1123, t1.Format(time.RFC1123)) line.

The test assumes that Parse, when called with layout time.RFC1123, will always be able to parse a value generated by Format(time.RFC1123), but this is wrong because even though RFC1123 uses the zone abbreviation (like MST), Format will fallback to the numeric abbreviation (like -07) if the current timezone has no abbreviation, but Parse is stricter and it won't accept -07 in the layout says MST when parsing back the time value.

IANA has been phasing out invented timezone abbreviations, and in the above snippet Format prints "America/Sao_Paulo" as -03, which is rejected by Parse when Parse is given a layout with the abbreviation MST.

A possible fix is to change the test to use time.RFC1123Z, which is like time.RFC1123 except the timezone is always in numeric form (like -03), which is guaranteed to work (unlike invented abbreviations, which as I said IANA is phasing out in newer tzdata releases).

Change https://golang.org/cl/52430 mentions this issue: time: use RFC1123Z in test to force numeric zone abbreviation

@jucie Any chance you could try out the patch at https://golang.org/cl/52430 and confirm it fixes the issue on your machine?

@jucie Or not, do not try it out. As pointed out it made the test too weak to be actually useful. I'll send a different patch.