What you need to do is the following:

1. Find out the locators for both the "Effective Date" element and the input "MM/DD/YYYY" element using xpath, css or ID

2. Click on the "Effective Date" element

3. Use the DriverWait class to utilize the wait based on "ExpectedConditions.ElementIsVisible" and "ExpectedConditions.ElementToBeClickable" methods on the "MM/DD/YYYY" element.

4. Then SendKeys to "MM/DD/YYYY" element

You do not need any arbitrary pauses, delays or sleeps