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Once in a while, the HP-75’s TIME and TIME$ commands return an incorrect value. For example, TIME$ might return “9:40:00” when in fact the time is 9:50:00 (off by ten minutes in this example). The error can actually be as great as one hour.

These erroneous time reads are due to a bug in the HP-75. The attached article explains the cause of this bug. It also explains how the HP-71B succeeded in avoiding this bug.

http://holyjoe.net/hp/The_Earthquake_Bug.pdf