TODO list for Perl module DateTime
- 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.