Variable: buffer-display-time
buffer-display-time is a buffer-local variable defined in buffer.c.
Documentation
Time stamp updated each time this buffer is displayed in a window.
The function set-window-buffer updates this variable
to the value obtained by calling current-time.
If the buffer has never been shown in a window, the value is nil.
Probably introduced at or before Emacs version 20.3.
Source Code
// Defined in /usr/src/emacs/src/buffer.c
DEFVAR_PER_BUFFER ("buffer-display-time",
&BVAR (current_buffer, display_time), Qnil,
doc: /* Time stamp updated each time this buffer is displayed in a window.
The function `set-window-buffer' updates this variable
to the value obtained by calling `current-time'.
If the buffer has never been shown in a window, the value is nil. */);