I will repeat what I have posted in response to your commentary in the blog:
I hope you mean that you want the alerts to appear 90 seconds ‘after’, not ‘before’, because otherwise it would be impossible to achieve. Your condition won’t delay anything for the first case of alert checking, because last_time is most likely 0 at the time of declaration. It will also work rather unpredictable after that – if the next alert triggers 60 seconds after the previous one, last_time will also be at least 60 seconds older than TimeCurrent(). You need to update time_current to current time only when the next new alert condition is met (but do not sound the alert at time yet).