Create a layout with your fields for entering a start and stop date. Let's call the layout "DateEntry".
Set Variable [ $leftcenter; value: Get (windowWidth ) / 2 ]
Set Variable [ $TopCenter ; Value; Get ( WIndowHeight ) / 2 ) ]
Set Variable [ $newWindowwidth ; Value: assign a value for the width of your new window in pixels here ]
Set Variable [ $NewWindowHeight ; value: assign a value for the height of your new window in pixels here ]
New Window ["Name of New window here" ; $NewWindowHeight ; $NewWindowWidth ; $LeftCenter - $newWindowWidth/2 ; $TopCenter - $newWindowHeight / 2 ; Dialog ]
Show Hide ToolBars [hide]
Go to Layout [//specify new layout for your window here]
The values of $newWindowHeight and $NewWindowWidth will depend on your design of the layout for this window. I often have to test the script a few times, adjusting the values of those two variables until I get a result that I like.