When creating a pivot table, how do I keep the original sort order and have the pivot group correctly.

I have daily flow rates of a river. I used the month function to determine the month and the year function to determine the year. I then used a vlookup to change the month number to a name (ex 1=Jan, 2=Feb, etc.) The cell no contains a formula to return for instance Jan 1972 and there will be 31 of those. I have a space added between the month and year. When I put this into a pivot, it sorts alphabetically so all the April's are at the top followed by the Augustsí etc. How do I make this in date order if the underlying data is not a date?

Maybe a better question is how do I calculate the monthly average of daily readings quickly. Certainly I could sit here for awhile and manually set up a table but shouldn't there be a way for excel to know that all the January 1989's, and March 2003's, etc should be grouped together.

Thanks for any help.