Denso BHT-7500S User Manual page 48

(2) When turning the BHT on after storage with no battery cartridge loaded:
Even after removal of the battery cartridge, the calendar clock will work with the
backup power source for a while.
If the calendar clock backup has stopped, loading the battery cartridge and
turning the BHT on will display the following message, prompting you to set the
current date and time.
Set the calendar clock according to the procedure given in Subsection 2.3.2.
(The indication "00/01/01 00:00" will vary depending upon the calendar clock
state.)
Set the current
date and time.
00/01/01 00:00
_ / / :
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