I have introduced a workflow rule that sends out an email based on time-based workflow rule. It starts sending out email when it still meets the workflow criteria 21 days prior to a datetime field.

My problem is that there are some records created before the introduction of the workflow rule (hence the time triggers have not been set)  and is halfway thru the 21 day period that were updated thus firing off the workflow rule and creating some time triggers in the past. Thus sending a ton of the same emails an hour or so after updating.

How do I prevent it from firing or at least changing this behaviour? Are there any workarounds?