Customer can change end time on a 30 minute booking and create an all day booking !?

Customer can change end time on a 30 minute booking and create an all day booking !?

I have been looking at changing start times for one of our resources, to add a couple more sessions.
In testing to see what the customer see, I have found that if they choose one of the 30 minute sessions, select one of the available start time, they also get the choice to select an end time? So they can either by accident or on purpose book all day! 
This cannot happen? 
I think that in the past, when they selected the start time, there was only one end time to choose from (default the end of 30 minutes!) 
Has something changed? or have I changed something that created this situation?  
thanks










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