date2name-prepend-date is a byte-compiled function defined in
date2name.el.

Signature
(date2name-prepend-date PATH TIME &optional WITHTIME)

Documentation
Prepend TIME to PATH.

If WITHTIME is not nil a timestamp in the format of
the variable date2name-datetime-format is used.
Otherwise a datestamp in the format of the variable
date2name-date-format is used.

References
References in date2name.el:
(defun date2name-prepend-date-write ...) 1 reference

Find all references Functions used by date2name-prepend-date

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Source Code
;; Defined in ~/.config/emacs/elpa/date2name-20190630.933/date2name.el
(defun date2name-prepend-date (path time &optional withtime)
  "Prepend TIME to PATH.
If WITHTIME is not nil a timestamp in the format of
the variable `date2name-datetime-format' is used.
Otherwise a datestamp in the format of the variable
`date2name-date-format' is used."
  (let ((directory (file-name-directory path))
        (filename (date2name-remove-date (file-name-nondirectory path)))
        (datestring (if withtime
                        (format-time-string (concat date2name-datetime-format
                                                    (date2name-choose-separation-character path))
                                            time)
                      (format-time-string (concat date2name-date-format
                                                  (date2name-choose-separation-character path))
                                          time))))
    (concat directory datestring filename)))