I'm trying to convert December 25, 1948 to epoch seconds.

Unless you have a Mac, epoch time is usually around Jan 1st 1970. You generally can't use negative epoch seconds, so dates before 1970 are out. A pre-OSX Mac can go down to 1904 — because that is its epoch.

Try one of the wider range date related modules, such as one using the Julian date. You can find a nice comprehensive list here.