In the olden days, we stored dates like e.g. 20180131121234, and do a
lexicographical check. With that, we could use e.g. upper-limits
201802312359 for "all dates in Feb 2018", even if Feb doesn't have 31
days.
However, nowadays we use time_t values, and g_date_time_new_local raises
errors for non-existent days; easiest fix is to massage things a bit; so
let's do that.
Fixes issue #1197.
6fe67b354d