Hello,
The better way to start your trigger working is to start the market refreshing at that time.
You can schedule this for each market X minutes before with Settings -> Monitoring options -> Scheduler.
By the way if you have a Market Locator template you even can run it on a periodic scheme to add markets automatically (the same settings tab).
If you need to run just a specific trigger at that time (or each trigger at different times) there is now_time and market_start trigger variables you can use for time calculations.