[DATA-2390] Document that DeleteTabularData uses UTC (viamrobotics#523)

proto/viam/app/data/v1/data.proto

@@ -290,9 +290,12 @@ message BinaryMetadata {
   repeated string dataset_ids = 9;
 }
 
-// DeleteTabularDataRequest deletes the data from the organization that is older than `delete_older_than_days`.
-// For example if `delete_older_than_days` is 10, this deletes any data that was captured up to 10 days ago.
-// If it is 0, all existing data is deleted.
+// DeleteTabularDataRequest deletes the data from the organization that is older than `delete_older_than_days`
+// in UTC time. For example, if delete_older_than_days=1 and the request is made at 1AM EST on March 11
+// (March 11 5AM UTC), this deletes all data captured through March 10 11:59:59PM UTC.
+// If the request is at 10PM EST on March 11 (March 12 2AM UTC), this deletes all data captured
+// through March 11 11:59:59PM UTC.
+// If delete_older_than_days is 0, all existing data is deleted.
 message DeleteTabularDataRequest {
   string organization_id = 1;
   uint32 delete_older_than_days = 2;