Somewhat related to:

Progress - When should I go from 100% to 0%, or vice versa?

I'm wondering if it's appropriate for a progress bar which is a countdown to go from right to left. That is, in this case I want to communicate urgency to the user that a task must be completed before the expiration of a certain amount of time. I thought perhaps it should be more like an hourglass draining. However, since it's non-standard, I'm wondering if it warrants breaking convention in this case or whether I should just stick to a standard LTR progress bar for familiarity.