I invite you to consider our Enterprise Stopwatch solution, which is designed to improve and expand your users’ adoption of Workfront by making it simple and convenient for everyone to track time from their browser, tablet, or smart phone, as the work occurs. Alternatively, our UberTimesheet solution takes more of an "after the fact" approach (similar to the native timesheets, but with several advantages).
Both store the hours entered directly and natively within Workfront, where you can then view and work with them As Usual.
@Mike Rubin‚ I don't have a suggestion on that one, but I do have a caution/feature suggestion. We had a tool like that at a prior agency I was with, and there was not a single month that we didn't have major time entry errors to reconcile because of it. People remembered to turn it on - but turning it OFF was another story. I imaging tools have since improved, but suggest making sure it has some kind of gating/auto-shut off feature for reasonableness as you evaluate options.