pom: Fix fencepost bugs.

pom: Fix fencepost bugs.

Under some conditions pom would report "waning" and then "full", show
higher percentages than it should, and get confused by DST.  Fix.

Before:

  2018.01.30: The Moon is Waxing Gibbous (97% of Full)
  2018.01.31: The Moon is Waning Gibbous (100% of Full)
  2018.02.01: The Moon is Full
  2018.02.02: The Moon is Waning Gibbous (98% of Full)

After:

  2018.01.30: The Moon is Waxing Gibbous (96% of Full)
  2018.01.31: The Moon is Waxing Gibbous (99% of Full)
  2018.02.01: The Moon is Full
  2018.02.02: The Moon is Waning Gibbous (97% of Full)

PR:                    231705
Submitted by:          Andrew Gierth
Approved by:           allanjude (mentor)
MFC after:             2 weeks
Differential Revision: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=231705