Nikon Z8 User Manual page 665

d5: Extended Shutter Speeds (M)
Extend the range of shutter speeds available in mode M; the slowest shutter speed available when
[ON] is selected is 900 s (15 minutes). Extended shutter speeds can be used for pictures of the night
sky and other long exposures.
When [ON] is selected, the time remaining in the current
exposure will be displayed in the control panel at shutter speeds
slower than 30 s.
At speeds slower than 1 s, the shutter speed displayed by the
camera may differ from the actual exposure time. The actual
exposure times at shutter speeds of 15 and 30 seconds, for
example, are respectively 16 and 32 seconds. The exposure time
will again match the selected shutter speed at speeds of 60
seconds and slower.
G button U A Custom Settings menu
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d5: Extended Shutter Speeds (M)