Restrict live an input field comparing it to another field

I have a php page and these two inputs fields. My goal is to check if the second input is equal to first input (live) and make an error msg. The code i have so far is this:

<div class="input-group date" id="timepicker1"  
                                                                
    data-target-input="nearest">
    <input type="text" 

    class="form-control datetimepicker-input"
    data-target="#timepicker1"

    name="FirstDate" id="FirstDate"  />

</div>
<div class="input-group date" id="timepicker2"  
                                                                
    data-target-input="nearest">
    <input type="text" onclick="CheckForErrors()"

    class="form-control datetimepicker-input"
    data-target="#timepicker2"

    name="SecondDate" id="SecondDate"  />

</div>

In the end of the php i have a Javascript checking:

<script type="text/javascript">
    function CheckForErrors() {
      // Check if First Time is equal with second Time
      if (FirstDate == SecondDate) {
        alert("Error MSG!");
      }
      return false;
    }
</script>

Its not working, as u understand. Any help ?
PS. I’m a begginer so please be patient 😀