Date Validation Business Rule failing

We have a business rule on our Appointment Entity that ensures that the End Date does not precede the start date.  Unfortunately, this seems to trigger when the Start Date is changed even though the End Date changes automatically to account for change in Start.

E.G

1. Appointment is created - Start Date/Time defaults to now (rounded up to 1/2 hour) - End Date/Time defaults to Start Date +30minutes
2. User changes Start to later date - End Date changes automatically to new Start +30 minutes
3. Error message (generated through Business Rule) shows and can only be cleared by changing End Date/Time (providing changed to after Start)

I can't see any reason why it should behave this way - any ideas?

Many thanks

Matt