TimeCard does not allow negative values?

My users are reporting they are no longer able to enter negative values for Actual Effort on the TimeCard page.  Occasionally time gets logged to an incorrect task, so when that happens they used to "back it out" by logging a negative value and then logging the time to the correct task.  Is there a new edit that no longer allows this?  Is the only way for them to correct time by directly manipulating the Actual Effort field on the task?

Thanks!