TODO list for Perl module DateTime

* leap seconds

- what's the relation between UT1 and MJD/JD ?
  see:   http://hpiers.obspm.fr/eop-pc/earthor/utc/leapsecond.html

* strftime method

- more tests for other languages

* sub-second resolution

- more tests: add/subtract/compare


* Other

- document RD days/seconds(/nanosecs?) in a separate document that
will be the reference for DateTime.pm internals, as well as being
useful for other calendar implementors.

- thorough tests for subtraction & addition overloading


NOTE TO FUTURE DEVELOPERS:

Attempting to implement add_duration in XS actually seemed to slow
date math operations down.  Sad but true.

Same for implementing _normalize_tai_seconds.  Apparently the calls
out to Perl from C overwhelmed any speed improvement.
