inst.

Inst. is short for instant, an old way of saying "of this month." A writer who dates a letter in June and mentions "your favor of the 4th inst." means June 4.

It turns up in letters, newspaper notices and court papers through the nineteenth century, often next to ult. (last month) and prox. (next month). To turn it into a full date you need the date of the document itself, so always note where the inst. sits before converting it.