//Using multiple assignment, you can distinguish a missing entry from a zero value.
var seconds int
var ok bool
seconds, ok = timeZone[someTimeZone]

//if someTimeZone doesn't exist, seconds will be set to 0, and ok will be set to false.

//Example:
func offset(tz string) int {
    if seconds, ok := timeZone[tz]; ok {
        return seconds
    }
    log.Println("unknown time zone:", tz)
    return 0
}

//If you just want to check for existence and don't want to assign a value, use the blank identifier.
_, present = timeZone[tz]