SimpleDateSerializer
Default KSerializer for SimpleDate - encodes as the string "YYYY-MM-DD".
Wire format example: "2082-02-14".
Year always pads to 4 digits, month and day to 2. Parses any year width >= 1 on the way back in (so historical Bikram Sambat years that take 4 digits, plus any one-off "99-01-01" test fixture, all round-trip).
The date is always interpreted as Bikram Sambat - no era tag in the wire form because the library doesn't model AD dates as SimpleDate.
See also
for a {year, month, dayOfMonth} JSON-object form.