What am I doing wrong with Particle.publish()? --> "Particle.publish() will require event scope to be specified explicitly..."

[ScruffR]

How about Particle.publish("StartTime", String(startTime), PRIVATE);
this would also line up with the docs
https://docs.particle.io/reference/device-os/firmware/photon/#particle-publish-

MY_DEVICES would be the scope for Particle.subscribe()

The opposite scope would be PUBLIC for publishing and ALL_DEVICES for subscribing.