average of certain data between two dates and times in Excel

I need to be able to average data between two dates and times.  In the example file, I need to (for example) average the Data in column D for the time period 10/13/10 7:36 - 10/13/10 7:40.

Id like code that I can use so that this can be accomplished in one cell - because I have to average many various dates and times this way.

Thanks in advance Book1.xlsx