Yesterday at midnight UTC is the start of the previous UTC day. It is a boundary, not a local clock reading. The live reference computes the current value; use the same definition in queries and scheduled jobs.
Prefer an exclusive upper boundary
WHERE event_time >= :yesterdayStart
AND event_time < :todayStart
This form includes every event in the intended period without inventing a last second or losing values with sub-second precision.
UTC and local days are different ranges
A UTC day begins at 00:00:00Z. A business day in a named timezone begins at local midnight, which can be a different Unix timestamp and can move when daylight-saving rules change. Decide which day the report means before calculating the boundary.