Capture time both at the start of each sensing cycle and at the end all sensors are read, before the Publish/SD logging is done. Publish and log both or send 1 plus the elapsed time difference.
Also consider using epoch seconds instead of ISO8601 FULL format for Publishes to save message space and bandwidth. Particle's Cloud API will already supply human-readable ISO 8601 cloud timestamps that will help for debugging.
Capture time both at the start of each sensing cycle and at the end all sensors are read, before the Publish/SD logging is done. Publish and log both or send 1 plus the elapsed time difference.
Also consider using epoch seconds instead of ISO8601 FULL format for Publishes to save message space and bandwidth. Particle's Cloud API will already supply human-readable ISO 8601 cloud timestamps that will help for debugging.