Wrong DST application

[loxs]

Originally reported here by Petter Urkedal

The calendar library seems to apply DST of the current date rather than the date to be converted. From a locale time zone with DST, the following should show the problem:

Printer.Calendar.to_string (on (fun () -> Calendar.make 2015 1 1 10 00 00) UTC ());;
- : bytes = "2015-01-01 12:00:00"
Printer.Calendar.to_string (on (fun () -> Calendar.make 2015 7 1 10 00 00) UTC ());;
- : bytes = "2015-07-01 12:00:00"

As I'm in a CET with DST time zone I would expect the first to return 11:00:00.

Calendar version 2.03.2.

[loxs]

Caused by relying on global state, depends on #8