how to show month first date and last date in label

hi
I have three label in my aspx page its called L1,L2,L3
for example:- on load L1 shows like this 10-Sep-2012
in label l2,l3i need to show like 01-Aug-2012  in l3 31-Aug-2012
how to do so..?
give me javascript or any other method...
Thanks
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