Subtract a text field from datetimefield

Subtract a text field from datetimefield

I am looking to subtract a date time field from a custom text field to return a time. We have a date and time we need to be at a location ( datetime field), and each location is a different time and distance from a branch (travel time). Our travel time text field is entered as it is entered as 00:00. how would I create a formula for this to work. An example is:

12/1/2014 6:00pm  travel time= 3:00    

How would get the result of 3:00 pm?

Please note I have hundreds of different date times for various customers and each customer is a different distance from he branch location.

I am not opposed to reformatting the leave time field if necessary.

Thanks