What I can do:
Get a subset of HL7 messages.
Send the messages to a RecordMap (with key fields from the HL7 message).
Save each message to a file.

How can I send all messages until a set time (e.g. 9am each day) to single file?

Scenario:
In the period 09:00 1 Jan 2022 to 09:00 2 Jan 2022 there are say 50 messages to save to file: File001.txt
In the period 09:00 2 Jan 2022 to 09:00 3 Jan 2022 there are say 40 messages to save to file: File002.txt
etc