Suppose you have been given a date say 10/22/14 (MM/DD/YY) and you want to calculate the first day of the Current Month. Hence, you want to achieve a result of 10/1/2014 (MM/DD/YY).
The formulas to be used –
=DATE(YEAR(A1),MONTH(A1),1)
=A1-DAY(A1)+1
=EOMONTH(A1,-1)+1