Currently there is no way to get the actual traffic counter values from within iRules. If you could, you could then use the "clock" command to get the timestamp differences. If you would like this feature, I'd recommend submitting a feature request to product support.
But, if you are more flexible in your reporting options, you could always write an application that uses our iControl management APIs to query the statistic counter values and process them externally. Check out the DistributionMonitor C sample in the SDK for an example of how to do this.
-Joe
Nimbostratus