You have absolutely no way of knowing that. He didn't pray publicly. That's all you know for sure.
In fact he stated it quite differently. The Washington Times, May 2009
In addition, Clinton held no National Day of Prayer events in the White House. GHWBush and Reagan each only hosted one during their respective administrations.
We have enough problems with the Obama Administration. This isn't one of them.
Well..you MIGHT be right....except for the ISLAMIC DAY OF PRAYER At the Whitehouse in 2009.....see post 20.