"month out of range" when parsing date without separators - date

Question

Any ideas how to get golang to properly parse a date string such as 31916
I keep getting a month out of range error.
date, err := time.Parse("1206", "31916")
fmt.Println(date, err)
Of course I want to treat the month as 3 and not 31 like it's doing, but I'm not sure how to force it to stop at 3 for the month outside of adding separators to the format.

Answer 1

For example,
package main
import (
"fmt"
"time"
)
func parseDate(date string) (time.Time, error) {
if len(date) == 5 {
date = "0" + date
}
return time.Parse("010206", date)
}
func main() {
date, err := parseDate("31916")
fmt.Println(date, err)
date, err = parseDate("031916")
fmt.Println(date, err)
date, err = parseDate("121916")
fmt.Println(date, err)
}
Output:
2016-03-19 00:00:00 +0000 UTC <nil>
2016-03-19 00:00:00 +0000 UTC <nil>
2016-12-19 00:00:00 +0000 UTC <nil>

Answer 2

You can't. The layout you pass is not deterministic - there is no way for Go to know when the month ends and the day starts in your string - besides when one option fails and the other one does not (think of "11106" - is that 1\11\06 or 11\1\06?). Your best bet is to write a wrapper which makes the choice deterministic:
import (
"time"
"strconv"
)
func parseWeirdLayout(dateString string) (time.Time, error) {
parsedString := ""
if len(dateString) == 5 {
month, err := strconv.Atoi(dateString[0:2])
if err != nil {
return time.Now(), err
}
if month < 1 || month > 12 {
parsedString = "0" + dateString
} else {
parsedString = dateString[:2] + "0" + dateString[2:]
}
} else if len(dateString) == 4 {
parsedString = "0" + dateString[:1] + "0" + dateString[1:]
}
return time.Parse("010206", parsedString)
}
Test here: https://play.golang.org/p/u1QFPzehMj
Or simply use a different, deterministic layout, e.g. "010206".