Thanks a lot for true event explanations, WP!
Finally I have used onEditingDone as the most appropriate event for date validating in this case.
Bart, thank You for ideas. I'm just satisfied with the simple solution achieved and don't want to get in more complicated ones. If You look just to the picture of the interface attached, You may guess that in most cases users will choose "Today", "Last week", etc. from Date range list. Only under occasional "Custom" choice Dates are to be entered manually and therefore somehow validated. Entered Dates are used for selecting data from DB.
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