Knowing when things happened.
WHY IT MATTERS
Readings without reliable time cannot be ordered, compared or analysed.
WHAT DEVICES TYPICALLY HAVE
A counter since power-on, and possibly a real-time clock.
WHAT A REAL-TIME CLOCK REQUIRES
A backup power source, to survive main power loss.
WHAT DRIFT MEANS
Clocks diverging from true time, at rates varying with temperature.
HOW MUCH
Seconds per day, on inexpensive hardware.
WHAT THAT MEANS OVER MONTHS
Minutes of error, which matters for correlation between devices.
WHAT SYNCHRONISATION OPTIONS EXIST
Network time protocols, where connectivity allows Time supplied by the network or gateway Satellite positioning, which provides precise time
WHAT TO RECORD
Both the reading's time and the time source's quality.
WHY
It allows downstream systems to judge reliability.
WHAT TO USE FOR STORAGE
A single time standard, without local offsets.
WHY
Local time complicates ordering and breaks at transitions.
WHAT TO CONVERT AT PRESENTATION
To the viewer's local time.
WHAT TO TEST
Behaviour across power loss, and after long disconnection.
