Time slider accessing only certain months of a year

[MichaelMitchell1]

I'm attempting to figure out how to go about using the dynamic time slider https://developers.arcgis.com/javascript/jssamples/time_sliderwithdynamiclayer.html and instead of allowing access to all days between 1921 and 2009 like this code:

 timeExtent.startTime = new Date("1/1/1921 UTC");
          timeExtent.endTime = new Date("12/31/2009 UTC");

I would like to only access months 1,2,3,9,10,11,12 between years 1921 and 2009.

Any thoughts?

Thanks