In doing some genealogical research, I’ve found dates recorded in a way I’ve not seen before, and I wanted to get some input about it, hopefully from someone who is familiar with it.

The format is as follows:

15 : 3 m : 1641

I quickly grasp in looking at it that the obvious “modern translation” would be 3/15/1641. But other sources which give the same information (in this case, a birthdate) say it should be interpreted as 5/15/1641. Combined with other examples I have which all correlate, I’ve figured out that they are counting months starting with March, because all the dates have been consistently interpreted as 2 months beyond what I would normally read it as. So far, I’m wrapping my head around this pretty well.

Here’s where it fails me: I have a date which is, for example, 25 : 12 m : 1641. So that would be 2/25/1641.

But then my head says, "no, if you’re counting starting from March of 1641, that would be 2/25/1642.

And then I give up, because why would you come up with a system like that? If you know what year it is, and it’s the same year system we use today, why aren’t you starting and ending your years on the same months we do today? Am I really ready to accept a calendar that starts 1641 with 11 m, then 12 m, then 1 m through 10 m?

Help?

ETA: Google Books link to page in historic book with examples aplenty.

And to amp up the maddeningness of it all, apparently, at some point they just stopped and started using a modern format.