Excel exponential notation problem

I have a cell with 20:02:00:60:45:17:09:73, I remove the : by using replace and it changes to 2.00201E+15. In the formula bar it shows as 2002006045170970 but in the cell it shows as 2.00201E+15. The number in the formula bar is inccorect, 2002006045170970 should be 2002006045170973. I've found this is automatic, Excel uses exponential notation for numbers larger than 12 or more digits. But why is it changing the last number from 3 to 0?

Because Excels (and most any other program that uses the IEEE double precision standard) is limited to 15 digits when representing numbers. Anything longer than 15 digits is truncated.

Do you need this represented as a number? You can keep all the digits if you make sure to convert it to text.