Looking into old birth, death, and marriage records, I ran across dates such oas "on the 12th instant". Looking this up it turns out to be shorthand for "the current month". Related terms:

• Ultimo (the previous month, from Latin ultimo mense, abbreviated ult)
• Instant (the current month, from Latin instante mense, abbreviated inst)
• Proximo (the next month, from Latin proximo mense, abbreviated prox)