Have a look at the Computed Field module. You can use it to define a new dynamic field for any content type, which can either be stored in the database or computed on the fly. Some more details about this module (from its project page):
Computed Field is a very powerful field module that lets you add a custom "computed fields" to your content types. These computed fields are populated with values that you define via PHP code. You may draw on anything available to Drupal, including other fields, the current user, database tables, you name it. (Feeling the power yet? :) ) You can also choose whether to store your computed field values in the database with other content fields, or have them "calculated" on the fly during node views. (Although you should note that Views use requires database stored values.) This field is literally the Swiss Army knife of fields. So start cooking up your PHP based values!
You may also want to look at its Community documentation, especially its child page about "Calculating a duration given a start and end time", here is a quote from it:
... create a new computed field to calculate the duration as a decimal number of hours (where 1.5 is 1 hour and 30 minutes).
PS: not 100% sure if it would apply in this case, but for a possible alternative to the Views PHP module (as suggested in another answer here), you may want to use an approach as explained in "44,497 People Are Wrong: How to NEVER Need Views PHP".