[Publius6961]

Tennessee math?
Married at 15, not 13, but is 3 years older than thought to be.

Math between exact years is an exact science.
Math between exact dates is not.
Apples and oranges.

She may have been "amost" 14 in the first instance, and "justed turned 15" in the other.

I'm sure Tennessee math is just fine.

Montana or Idaho? Not so much. One of them wanted to redefine pi by law around 100 years ago.

[TomGuy]

Your math might work — if they also changed her birthdate.