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Set date and time in Open Firmware System
I was googling around for this and came up empty handed, so maybe I can save somebody else some time ... I wanted to set the machine clock of a PowerBook G3, and didn't want to go through the hassle of booting to a usable system so I could use System Preferences. Also, I was unsure of whether or not my machine was saving the real time clock between reboots, so I needed a way to set the clock after powering on, but before booting into OS X.

Reboot your machine, and hold Command-Option-O-F during the boot process to access the Open Firmware prompt. Most likely, the real time clock is aliased in OF as rtc, so the command is this:

decimal dev rtc sec min hour day month year set-time

So to set the clock to 10 seconds past 11:15PM on December 31st, 2005, the command would be:
decimal dev rtc 10 15 23 31 12 2005 set-time
The Open Firmware prompt should type ok after this, and you can then boot the Mac by typing mac-boot and pressing Enter.
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Set date and time in Open Firmware
Authored by: mike666 on Aug 15, '06 10:53:38AM
Hmm, I've always use the time&date command in OF (time&date hh:mm:ss dd/mm/yyyy). Although there seems to be a difference between what these two commands set - as if there are two times that are kept. I guess this would explain why on OS9-capable machines, there can sometimes be a major discrepancy between the time when booted to OS9 vs. booting to OSX. Can anyone enlighten?

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Set date and time in Open Firmware
Authored by: mike666 on Aug 15, '06 10:59:02AM
Correction: that command should be time&date hh:mm:ss mm/dd/yyyy

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Set date and time in Open Firmware
Authored by: Tupring on Nov 18, '07 02:16:32AM

I tried this just now on a Tangerine iMac and the date says it's June 28, 2002
(Yes, I entered today's date)

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