Time Tracking in AgilePlace?

We’ve got a customer who would like to have their AgilePlace users track time (actual effort) on cards. Are there any creative solutions for this? I know we can set up a custom numeric field and just have the users manually add their hours to any value already there. For instance, if three hours have already been logged on a card, and I work two more hours on that task, I could update the custom field’s value from “3” to “5”. But this seems like it would be prone to human error. Has anyone devised a better solution to tracking actual effort on cards?