jquery prevent textbox clear

i'm using the jquery masked input plugin
http://digitalbush.com/projects/masked-input-plugin/

In my jquery code i currently set the date mask like this and it works fine:

$('.StartDate').mask("99/99/9999");

I have 3 textboxes who have class="StartDate" set on them.
So the mask gets applied to the three textboxes.

On page load they have default values of

10/12/2017
10/13/2017
10/14/2017

But I noticed that when I click the left mouse button in one of the textboxes. The default values gets cleared out from the other two textboxes.

Do you guys know what jquery event gets fired when I click inside a textbox with the left mouse button?

How do I prevent the textboxes from clearing the default values ?