Time Fractions And User Input

What I am doing is creating a spreadsheet solution to tack race times. These show up in past performances in the following formats in paper copies: 45 2/5 seconds. The end user would have to put in maybe 30 of these times based on the number of contestants. Keeping in mind that the 45 2/5 number will be used in some calculations, I want to give the user the easiest input interface. Meaning instead of inputting like this; 0:45.4 (mm:ss/00 format), I would like them to just enter 45.4. the spreadsheet would input format this cell into a time format that would be able to have calculations done to it. Another example might be 1:35.60 (mm:ss.00 format) which is 1 minute 35 and 3/5 seconds. This would be entered as 135.6.

A few questions first:

1) Would it be easier to leave the number inputed alone until the calculations ar finished then translate into a time factor?

or

2) would it be necessary to convert into a time factor as inputted be cause of the 60 factor involved with the time factor?

Having asked those questions, setting this all up with the exact scope I am working towards, any help or sugesstions as to the best way to proceed would be fantastic!