Handling alert messages of JS in CRM

I've written a funtion to enable or disable the datetime field depending on the two option value. If the value selected is yes, then the date time field is enabled or else it's disabled. If it's enabled then it becomes a mandatory field. There some validation conditions for the datetime field. If the conditions are not satisfied, then it pop's up a message and clears the datetime field. If the user selects a value using the calendar, then there is no issue. If the user enters the value which doesn't satisfy the condition and clicks on save button then it is showing two messages one which doesn't satisfy the validation and the other to enter the value as it is mandatory field. How to show only one message instead of two. The mandatory message is being showed by CRM.