I would like to have my git timestamps in UTC. Currently attempting to commit using magit gives

Wrong type argument: sequencep, my/magit-timestamp-utc

Here's the code:

(defun timestamp-now()
  (format-time-string "%Y-%m-%dT%H:%M:%S%z" (current-time)))

(defun timestamp-utc ()
  (let* ((original-tz (getenv "TZ")))
    (setenv "TZ" "UTC0")
    (let ((timestamp (timestamp-now)))
      (setenv "TZ" original-tz)

(defun my/magit-timestamp-utc (&optional prompt default)
  (concat "--date=" (timestamp-utc)))

(setq magit-commit-popup
  '(:variable magit-commit-arguments
    :man-page "git-commit"
    :switches ((?a "Stage all modified and deleted files"   "--all")
               (?e "Allow empty commit"                     "--allow-empty")
               (?v "Show diff of changes to be committed"   "--verbose")
               (?h "Disable hooks"                          "--no-verify")
               (?s "Add Signed-off-by line"                 "--signoff")
               (?R "Claim authorship and reset author date" "--reset-author")
               (?u "UTC" my/magit-timestamp-utc))
    :options  ((?A "Override the author"  "--author=")
               (?S "Sign using gpg"       "--gpg-sign=" magit-read-gpg-secret-key)
               (?C "Reuse commit message" "--reuse-message="
    :actions  ((?c "Commit"         magit-commit)
               (?e "Extend"         magit-commit-extend)
               (?f "Fixup"          magit-commit-fixup)
               (?F "Instant Fixup"  magit-commit-instant-fixup) nil
               (?w "Reword"         magit-commit-reword)
               (?s "Squash"         magit-commit-squash)
               (?S "Instant Squash" magit-commit-instant-squash) nil
               (?a "Amend"          magit-commit-amend)
               (?A "Augment"        magit-commit-augment))
    :max-action-columns 4
    :default-action magit-commit))
  • 1
    By the way, a better way to temporarily modify TZ would be: (let ((process-environment (cons "TZ=UTC0" process-environment))) (timestamp-now)).
    – npostavs
    Apr 20, 2019 at 12:10

2 Answers 2


I would like to have my git timestamps in UTC.

To just do this for all magit calls, put this in your init file:

(eval-when-compile (require 'cl-lib))
(with-eval-after-load 'magit
  (cl-callf setenv-internal magit-git-environment "TZ" "UTC0" nil))

Below is some code to make this toggleable from the commit transient command, which I wrote as an exercise to learn about transient:

  (require 'subr-x))

(require 'magit-git)
(require 'magit-transient)

(defclass magit--git-environment-variable (transient-variable)
  ()) ;; No slots needed

(defclass magit--git-environment-variable:toggle (magit--git-environment-variable)
  ((toggle-value :initarg :toggle-value)))

(defun magit--git-environment-variable-get-value (transient-variable)
  (getenv-internal (oref transient-variable variable) magit-git-environment))

(cl-defmethod transient-infix-set ((obj magit--git-environment-variable) value)
  (cl-callf setenv-internal magit-git-environment (oref obj variable) value nil)
  (unless (or value transient--prefix)
    (message "Unset %s" variable)))

(cl-defmethod transient-infix-set ((obj magit--git-environment-variable:toggle) _value)
  (cl-call-next-method obj (if (magit--git-environment-variable-get-value obj)
                             (oref obj toggle-value))))

(cl-defmethod transient-format-value ((obj magit--git-environment-variable))
  (if-let ((value (magit--git-environment-variable-get-value obj)))
      (concat "("
              (propertize (format "%s=%s"
                                  (oref obj variable)
                                  (car (split-string value "\n")))
                          'face 'transient-value)
    (propertize "unset" 'face 'transient-inactive-value)))

(cl-defmethod transient-infix-read ((obj magit--git-environment-variable:toggle))
  (if-let ((value (magit--git-environment-variable-get-value obj)))
      (oref obj toggle-value)

(define-infix-command magit-commit-env:TZ ()
  :class 'magit--git-environment-variable
  :description "Timezone"
  :variable "TZ")

(define-infix-command magit-commit-env-toggle:TZ=UTC0 ()
  :class 'magit--git-environment-variable:toggle
  :description "Timezone"
  :variable "TZ"
  :toggle-value "UTC0")

(transient-append-suffix 'magit-commit "-s"
  '("-u" "Timezone" magit-commit-env-toggle:TZ=UTC0))

You can of course just redefine the complete popup but a better way would be to just add a new switch using magit-define-popup-switch as documented in the manual. Edit: Magit now uses transient instead of magit-popup, the relevant documentation now is Modifying Existing Transients.

Regardless of how you define it, the "name" of the argument has to be a constant string. In some rare cases a switch has the form --arg=val like you are trying to do here, but it still has to be a constant string, not (the name of) a function that returns such a string.

An function that reads a value from the user can be added to an option. However that function is only called when the user invoke the option and is supposed to then read a value in the minibuffer. Automatically generating a value for an option every time the popup is invoked is not supported.

To achieve what you want you have to advice magit-commit-create and likely a few other magit-commit-* commands.

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