Query Max / Min Value in InfluxDB between now and the start of day


I’m recording the temperature of my house in the Influx DB database and I would like to know, during the day, the MAX and MIN value between the start of day until now ( and now to use now - 1day).
But the time operator “start of day” doesn’t exist in InfluxDB, but I can see this value on grafana, and I tell me that is possible to do that but why .
Could you help me please ?

Thank you !