The 82182A Time Module
The time module added a real-time clock. The clock allowed you to use the actual time in your programs.
You could also define alarms that triggered calls to user programs. The alarms were able to switch the calculator on, so it was possible to create programs that executed at some point during the day, then switched the calculator off until the next alarm. As the HP-41 was often used as data-gathering device in labs, this allowed the calculator to read data from monitored devices at specific times every day, without having to run in between the readings (thus saving battery capacity).
The HP-41CX had the time module built-in.
Read more about this topic: HP-41 Extension Module
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