How do i apply validation in widget level when datatype is Date?data

Configuration properties do not currently support options to make them required. Some potential solutions include:

1. When a date is not specified, use a default date (such as today).
2. When rendering the widget, if the date is not specified, render an error from the widget (you can also do this only in preview mode so it doesn't affect other users... just the page editor).

There are several conditions that are mention below:
1. That date will decide by admin only as per Event. We can't set a default date every time.
2. Every time admin will enter a start date and End date but there is a chance of wrong input (Such as admin can put past date in the end date section or forget to enter the End date)
That's why I need validation on widget level.

There are several conditions that are mention below:
1. That date will decide by admin only as per Event. We can't set a default date every time.
2. Every time admin will enter a start date and End date but there is a chance of wrong input (Such as admin can put past date in the end date section or forget to enter the End date)
That's why I need validation on widget level.

Why can't you just use the Event calendar?

Any validation can also be performed by the widget when it reads the configuration to act on it. That would be mostly similar to my recommendation #2 but with whatever rules you want to implement. 

Can you provide me any sample code for show error on Widget level only?

Yes, you'd check if you're in preview mode and render an error vs. not rendering if the configuration is invalid and the widget is being shown outside of preview mode:

#set($isValid = false)
## TODO: Identify if the configuration data is valid:

#if (!$isValid)
    #if ($core_v2_widget.IsInWidgetPreviewMode)
        <div class="message error">
            Error message here. This will only be shown in preview mode when someone is editing this widget.
        </div>
    #else
        ## the configuration is invalid, but we're rendering to an end-user so let's just hide the widget.
        $core_v2_widget.Hide()
    #end
#else
    ## TODO: Valid configuration, so let's render the widget...
#end