Checking the form field values before submitting that page
While you have a return value in checkform, it isn't being used anywhere - try using onclick="return checkform()" instead.
You may want to considering replacing this method with onsubmit="return checkform()" in the form tag instead, though both will work for clicking the button.
You can simply make the start_date required using
<input type="submit" value="Submit" required />You don't even need the checkform() then.
Thanks
use return before calling the function, while you click the submit button, two events(form posting as you used submit button and function call for onclick) will happen, to prevent form posting you have to return false, you have did it, also you have to specify the return i.e, to expect a value from the function,
this is a code:
input type="submit" name="continue" value="submit" onClick="**return** checkform();"