semi monthly date formula help please

I need help with a formula.

I would like to plug in any date (5/25/2011 or 7/15/2011 or any date) and find the first date of the semimonthly pay period following the date I plug in.

if I plug in 6/24/2011, the first day of the following pay period should be 7/1/2011, if I plug in 7/15, the first day of the following pay period should be 8/1/2011.

Can anyone offer help?
Thank you!

(If you're up for the challenge, I'd also like to find the last day of the same pay period.)