creating a date field for date range?
I have an idea!  How about if you add an additional field to your Job content type called 'Time at job' that's simply a text field.

Then, create a new rule with the rules module that fires off when a Job node is saved or modified.  In that rule, you can simply set the 'Time at job' field by calculating its value from the values of the two date fields (using php).

For the php code calculating the length of time between two dates, there are countless of tutorials online.  Here's just one that's quite comprehensive: https://stackoverflow.com/questions/676824/how-to-calculate-the-difference-between-two-dates-using-php.

Would that fit your scenario?  Calculating a third field upon node save/edit and displaying that instead?