Can I show an average for a "time to convert issues" report

[mdeancherry]

I've got a report that shows how long it's taking our account managers to convert a request to a project. The "Time to Convert" column was created in text mode:

displayname=Time to Convert
linkedname=direct
querysort=entryDate
valueexpression=ROUND(DATEDIFF({entryDate},{actualStartDate}),2)
valueformat=HTML

I don't get an "average" option for the column, like I do if I use a standard column selected from the drop down. Is there any way to display the average for the column other than exporting to Excel? I'm using groupings based on the different account managers, if that makes any difference. 

Thanks.

[skyehansen]

I think your most satisfying option will be to make a calculated field with your expression above, and then average based on that field.