Adding a position, how it effects Time to Fill

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We are struggling with our time to fill metric, especially when a position is added to a requisition during mid recruiting for that position. We currently use First approval date to offer accept date as the time to fill "window", but when a second position is added, the clock started back when the req was first approved and we didn't know we had to recruit for a 2nd position.

How do other companies handle this?

Thanks!