The DatePicker widget does not focus the input element when the calendar icon is clicked. Because of that, validation errors are persisted even when the value of the input has been filled out correctly. The input needs to be manually focused and then unfocused to get rid of the validation message.

This behavior can be observed on the following demo page:

https://demos.telerik.com/kendo-ui/validator/custom-validation 

The demo should be viewed in mobile mode.