I want to input times that would be displayed in the formats below, depending on whether the digits are needed or not, like this.

s.mss
ss.mss
m:ss.mss
mm:ss.mss

My main problem is being able to use them to do math with (sum and subtraction only).

• How could I do that?
  

Look at the

Explanation

1 - Format the number in this case Date/Time with