Normalizing two-digit years

[aantron]

Thank you for this library! It seems exactly what is needed in aantron/dream#270.

I tried round-tripping the three formats from the HTTP RFCs. For the RFC 850 format, with a two-digit year, the round-tripping doesn't quite work. The resulting date has the two-digit year. When printed in the preferred format,

Sat, 06 Nov 0094 08:49:37 GMT

Assuming these years refer to the 1900s (and since this format is obsolete), I had to write a function

let normalize_year ptime =
  let ((year, month, day), time) = Ptime.to_date_time ptime in
  let year = if year < 100 then year + 1900 else year in
  Ptime.of_date_time ((year, month, day), time) |> Option.get
  (* Option.get is safe here because this function updates the year. If the date
     was valid to begin with, it will be valid after the update. From @beajeanm:
     an additional consideration here is whether a two-digit year Y is a leap
     year if and only if Y + 1900 is a leap year, and we believe it is, as
     suggested by testing Ptime itself. *)

to normalize the year.

Should this be done in http-date? Is this something for the user or framework to decide? I can open a PR if appropriate.

[bikallem]

Is providing a century parameter feasible? i.e.

# Http_date.decode ~century:19 "Sunday, 06-Nov-94 08:49:37 GMT" |> Http_date.encode;;
- : string = "Sun, 06 Nov 1994 08:49:37 GMT"

If not then a PR for normalize_year function as above should be okay.

[aantron]

A century parameter seems pointless. Wouldn't this always be 19 for dates in this format?