Excel Question

i am making a excel sheet and i want to update the dates automatically everyday.
What is the formula for this……..???

like i have 30 rows….in my excel sheet..that 30 rows contains the data of the past 30 days……now what i am looking for is…. I want to update the for 30 rows by last 30 days date….Is there any formulae we can write on first colum and then we can drag the rest of the rows and put fill series so that we can get all colums filled in with the last 30 days date……just as like we do and fill S No: in colums…..