In genealogical and historical research it is good to know the meanings of the oft-used instant, proximo, and ultimo in regard to months of the year. I seem to now have short-term memory and usually look them up to be sure when I run across a reference to them in old papers, books, etc.
Instant means the present month. Older Use. n. the present or current month. adj. of the present month: your letter of the 12th instant. Often abbreviated inst. If someone refers to “your letter of the 12th instant” as in the Dictionary.com example, it means the 12th of the same month (i.e., if the statement is made in January, it is referring to “your letter” written in January).
Proximo adv. in, of, or during the next month: on the 10th proximo. Sometimes abbreviated prox. If someone refers to an event on the 10th proximo, it means the 10th of the next month (i.e., if the statement is made in January, it is referring to an event occurring in February).
Ultimo adv. in or of the month preceding the current one: on the 12th ultimo. Sometimes abbreviated ult. or ulto. If someone refers to a death on the 12th ultimo, it means the 10th of the preceding/past month (i.e., if the statement is made in January, it is referring to a death that occurred in December).
Hope this helps.
Ultimo and proximo are Latin, from ultimo mense (previous month) and proximo mense, (next month). Instant seems to be from the English word instant, in its meaning of “current.”