From given date how many days happen in a month

Hi Experts,

I have a small query in my report i.e From given date how many days happen in a month

date= current date

26/12=26/12 means 25 days happen.

12/12=12/12 means from 1st of month to today 12 days happen

i need a formula for this i tried it but unable to crack it so please help me

rest how many days i need to calculate

Thanks & regards,

vinay.G