BigQuery firebase_performance partition not matching timestamp

Question

I have my Firebase project setup to export Performance data to a BigQuery table which is partitioned by DAY. When I query all records within a specific partition, it's returning records with timestamps for different days.

Query:

SELECT 
  event_timestamp,
  _PARTITIONTIME
FROM `{project ID}.firebase_performance.{table name}` 
WHERE TIMESTAMP_TRUNC(_PARTITIONTIME, DAY) = TIMESTAMP("2023-08-09") 

Sample results: | event_timestamp | _PARTITIONTIME | | --- | --- | | 2023-08-06 08:30:51.647951 UTC | 2023-08-09 00:00:00 UTC | | 2023-08-08 14:01:06.485670 UTC | 2023-08-09 00:00:00 UTC | | 2023-08-09 22:35:37.438460 UTC | 2023-08-09 00:00:00 UTC | | 2023-08-10 06:59:55.075843 UTC | 2023-08-09 00:00:00 UTC |

I could understand if the timestamps were within 24 hours of each other and say that it's due to time zones, but I'm seeing records for wildly different days. Why is this?