From http://yaml.org/type/timestamp.html

Regexp:

 [0-9][0-9][0-9][0-9]-[0-9][0-9]-[0-9][0-9] # (ymd)
|[0-9][0-9][0-9][0-9] # (year)
 -[0-9][0-9]? # (month)
 -[0-9][0-9]? # (day)
 ([Tt]|[ \t]+)[0-9][0-9]? # (hour)
 :[0-9][0-9] # (minute)
 :[0-9][0-9] # (second)
 (\.[0-9]*)? # (fraction)
 (([ \t]*)Z|[-+][0-9][0-9]?(:[0-9][0-9])?)? # (time zone)

The first form (ymd) does not allow single digit days or months, but the regex in js-yaml/type/timestamp will allow this.