PDL::DateTime is a subclass of PDL piddle for storing date-time values (scalar
piddles, vectors, matrices or generally ND-piddles):
- its PDL type is always LongLong (64-bit signed integer)
- stored values are microseconds since 1970-01-01T00:00:00.000000Z (can be both
  positive or negative)
- it is still a piddle so you can do all usual PDL arithmetic + PDL::DateTime
  defines some new methods (see below)
