如何在提交前比较两种类型的datetime输入

I am working on a web project using j2ee. anyway, in my jsp page I have a form containing two inputs of type datetime-local, and one of them must be greater than the other,it's like validating start and end datetimes, so I don't know how to make this test before submitting..I tried a javascript code in submit button onclick, I tried to make the input field of the second date empty if it is lower than the other,but it doesn't work. can anyone help me?I am working on a web project using j2ee. anywa