I was writing a query that requires the sum of a field since midnight through to now, and there isn’t an obvious function for it. This is a quick fix for queries that require a today() timeframe via the truncate method. Obviously, you can adjust the units to make it even more useful. See the official InfluxDB documentation.
import "date"
from(bucket: "MyBucket")
|> range(start: date.truncate(t: now(), unit: 1d))