Find top and average value from input field text
The PHP function strtotime() might help you here:


  strtotime — Parse about any English textual datetime description into a Unix timestamp


For example, you could create a computed field and use that function to convert the text input to a time value.

But as you're using a text field and not a date/time filed, I doubt that you'll be able to get all values correctly (because user input can differ enormously).