Birth date coded as number of months since a given date

[natalia carralero]

I have a date of birth variable coded as the number of months elapsed since January 1900. So, for instance, for a person born on January 1901, this variable would take the number 12.
From this variable, I need to create the actual date of birth, but I am unsure of how to do it efficiently. Any ideas?
Thank you!

[Leonardo Guizzetti]

Here is an example. Begin at the "Start here". I set up some trivial data to show that the monthly dates you have can be converted to the calendar birth month by adding back in the offset date (Jan 1900). I am assuming that the number of months you have is already a numeric variable.

Note: you cannot accurately create a date of birth, because your data are coarsened up to the level of a month, hence my use of birth months, and not birth dates.

input str7 mob
1901jan
2023may
end
label var mob "month of birth"

gen birthmonth = mofd(date(mob, "YM"))
format %tm birthmonth

gen have = birthmonth - tm(1900jan)

* start here
gen want = have + tm(1900jan)
format %tm want