In Excel there is a very useful function called EOMONTH which creates a date at the end of a month (hence the EO part of the function name). So if you give it any date e.g. 16 August 2014 and ask it for EOMONTH and specify a month later it will generate 30 September 2014. But what about an Excel first day of month function or formula?
At present these is no BOMONTH. But there is an easy way to do it using the EOMONTH function and basic arithmetic.
Create your formula so that the end result is the end of month of the month before the date you want. Then add 1 to the date.
So for example if I have the 16th August 2014 in a cell and I want to show it as the 1st September 2014 I would create the following EOMONTH function. Assuming that cell B2 contained the date 15/08/2014 the formula would read =EOMONTH(B2,0)+1.
This is the equivalent of an excel first day of month function.
For more on dates you can either get the Dates ebook or attend an Advanced Excel class