When entering numbers with more than 15 digits all the digits after the 15th
place automatically change to zeros.

It doesn't matter what the colum width is, the cell format is number, 0
decimal, no commas. If I change the format to text all the digits hold as
entered. Why don't they hold as entered when formated as numbers

example: I enter 1231231234567123456 the number changes
to 1231231234567120000