Create a rule
 

I have a spreadsheet that is sort of a schedule for events for a Drill Meet.
Across the top are the events and down the left side are times from 9 am to 4
pm in 15 minute increments (i.e. 900, 915,945, etc.).
In the cells will be the names of the schools participating in the meet so
that they know which school does which event at what time.
What I need to be able to do is to make sure that no team can be put into an
event within 29 minutes (because 30 minutes apart IS ok) of their previous
event... sort of like a "rule" that says, for instance: we have school A
scheduled for unarmed drill @ 900 and then happen to pop school A into armed
drill @ 915, it will automatically tell me that there is a problem.
I know I should know how to do this, but for the life of me I can't remember
how!

Your help is greatly appreciated!
Thanks in advance...

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MaggieB