I am facing some problem with Start date and end date Suppose If by mistake some one enters "End date" < "Start date" then the calendar shows unusual behavior I can understand that but how to avoid saving the incorrect value of "End date" this might be the basic step but I am a learner Please help

I think the nicest way to do this is to use a script trigger on all 4 fields (both date and time fields). We'd us an OnObjectValidate trigger and have it run a new script that simply checks if the start date / time is after the end date / time. Your script can warn the user and, perhaps, change the start or end time fields as well (in case your users ignore the warning).

Hope that helps,

John