The hidden date of a geocache is the date that it was hidden. A cache owner sets this date when creating the cache listing.
Jasmer / Hidden month
The Jasmer, or hidden month statistic, is a grid to find caches hidden in different hidden months since the first cache was hidden in May 2000. It was named after the owner of the first challenge cache set with this requirement (GC1GBC1). There is also a related BadgeGen badge.