Timed macro operation needs more accuracy

Hi...this board is great.
I have a macro that publishes to the Internet every 5 minutes.

the last string before publishing is set up as the standard:

Application.ontime now+TimeValue ("00:05:00") "name of publishing macro"

This runs all day with stock market analysis output and starts automatically when the market opens and stops when it closes, but the 5 minute update slips time throughout the day,with the slippage a product of processing time which changes depending on how busy the market is.

I would like to make sure that when I publish the command is sent relative to the clock every 5 minutes, not from elapsed time since the macro was started...

Would appreciate any guidance...

Thanks in advance,
Bill Ford