Store both timestamps
Record the measurement timestamp on the node and the arrival time at the server. When the gateway reconnects, a reading can arrive now even though it was measured minutes ago.
Show the measurement time and source
Use the measurement timestamp to mark a reading as stale. Backfill charts with delayed readings and retain source labels so operators can identify live and simulated values.
Test an outage and recovery
Record a normal sequence, disconnect the gateway, and reconnect after readings have buffered. Verify that delayed points return to their original positions and that the current value stays stale until a fresh sample arrives.