Problems with front end .csv upload of duration values

I have an app where people upload data which is then transformed through a series of workflows and displayed. One of the fields is a duration value. When I set the Field Type to number or date, it throws an error, so I have them upload the field as text and it handles it. This is where my problem comes in, I need to transform that value back into a duration value that can be used in calculations. When I use the convert to number action, it takes 00:08:45 and turns it into 845. Any help would be greatly appreciated!