Nikon D780 DSLR Camera (Body Only) - Manual (Page 656)

610 Menu Guide > A Custom Settings
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 in other situations that require that the
shutter remain open for long periods.
• To use extended shutter speeds, choose [ On ] and select a
shutter speed of 60 s or slower.
d6: Extended Shutter Speeds (M)
G button
U A Custom Settings menu