fix NMEA date fiddling. (#437)

fix NMEA date fiddling. (#437)

Three errors were corrected:
1) SECONDS_PER_DAY is assumed to be an integer type.  It was recently
errorously changed to to double.
2) when backfilling dates we need to subtract a day at rolloever,
not addit.
3) If we have a date, then it came with a time.  Don't set the backfill
time to 23:59:59.

Add test coverage for the above.