Start dates in excel vba

The below code doesnt give the first date of the month, while if i just use the below date function in a excel sheet It works well. Please help
whats wrong in the below code. For example if input in A1 15/05/2013. It should give value in C1 as 01/05/2013

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Range("c1").Value = DateSerial(Year(Range("a1").Value), Month(Range("a1").Value), 1)