Relative/Dynamic lookback time for alerts in InfuxDB

I am sending events to InfluxDB which are timestamped. These are basically events to external service. I wanted to set an alert which would execute if the count of such events from 12 AM today to now() extends a certain number, an alert should be raised.

I have checked out Influx but it seems that you can lookback only some constant time such as 5 mins, 10 mins, 1 hr, 12 hr, 1 day etc from now(). But for me the lookback is dynamic(start of the day).

Please help with some pointers to achieve this.