Minor optimization: instead of converting between days and years using loops
that start in 1970, assume most conversions are going to be for recent dates
and use a precomputed number of days through the end of 2016.

This is a do-over of r320997, hopefully this time with 100% more workiness.

The first attempt had an off-by-one error, but instead of just adding
another mysterious +1 adjustment, this rearranges the relationship between
recent_base_year and recent_base_days so that the latter is the number of
days that occurred before the start of the associated year (instead of the
count thru the end of that year). This makes the recent_base stuff work
more like the original loop logic that didn't need any +1 adjustments.