Epoch/unix timestamp

We have a party that wants to communicate the timestamps of the messages in epoch/unix format. eg, they want to send as timestamp the number of seconds since January 1st 1970.

Based on other posts in the community, I tried to implement this as followed in the transform:

start: ~ expression "dateTime('1970-01-01', 'yyyy-MM-dd') + period(utc_timestamp,'SECOND')"

where utc_timestamp in the epoch timestamp.

This seems to work to create the measurements from it, but the performance seems to be terrible. When I have this in my transformation, the transformation takes about 3,5 minutes for each measurement.
When hardcoding the date, and as such dropping the addition of the period, the transformation ran fine.

Is there anything to explain why this is running so slow? Is there a better way to read an epoch timestamp and transform it to the internal date format?

Following up on this behavior: this was fixed starting version 7.8.