How to display date in Julian in the mode line?

Question

The following setting display the date in Gregorian calendar in the mode line.

(setq display-time-day-and-date t)
(display-time)

My question is how to change the above settings for date to be shown in the Julian calendar?

Answer 1

You can compute arbitrary date strings by overriding display-time-string-forms.

The documentation says this:

List of expressions governing display of the time in the mode line. For most purposes, you can control the time format using `display-time-format' which is a more standard interface.

This expression is a list of expressions that can involve the keywords load, day, month, and year, 12-hours, 24-hours, minutes, seconds, all numbers in string form, and monthname, dayname, am-pm, and time-zone all alphabetic strings, and mail a true/nil value.

For example, the form

'((substring year -2) "/" month "/" day
  " " 24-hours ":" minutes ":" seconds
  (if time-zone " (") time-zone (if time-zone ")")
  (if mail " Mail" ""))

would give mode line times like `94/12/30 21:07:48 (UTC)'.

To display the Julian date as provided by calendar try this:

(require 'calendar)
(setq display-time-string-forms
      '((calendar-julian-date-string)))
(display-time)