While most instances of “instant” may be easy to discern the meaning from the context, my encounter with this unfamiliar usage was quite befuddling to me.
It was from a 19 October 1904 wedding announcement:
Miss Bessie Cleghorn and Mr. Forrest Additon of Flowery Branch, were married by ‘Squire Walter W. Cooper 4th. instant. The wedding created some surprise among their friends, but they have the best wishes of all.
The Gainesville News, v. 16, n. 22, 19 Oct 1904, page 1
What is “4th instant”?
While I could not figure out a Google search that would reveal an explanation of this particular usage, a quick search in older newspapers showed examples that made is clear.
This usage of “instant” indicates a day in the current month or of the current month. The Oxford Languages definition served up by Google labels this usage as “Dated” – an up-to-date way to say “archaic” or “obsolete”!
It was abbreviated as “inst.” There are also the terms “ultimo” for the previous month and “proximo” to refer to the next month. These were abbreviated as “ult.” and “prox.” respectively.
When was it used?
I found it in newspapers at least as far back as 1763 – the newspaper clipping above is from the 18 October 1764 Georgia Gazette, back when they still used an “f-like” character for the letter “s” – or at least the “long s” . I probably would find “instant” used this way further back if I also searched other newspapers, such as European ones.
It seemed to fall out of usage shortly after the 1904 article cited above. I found a few references as late as 1910, searching in newspapers from Georgia. It seems to have continued in business correspondence for a bit longer, perhaps a late as the 1950s in some circles.
Conclusion
I think this may have come into usage to save space and type.
It could have fallen out of usage because of the need to confirm the date the document was written in order for it to make sense and be accurate.
I use it as a reminder to consider the times in which the writing originated, and how change is always with us.