Variable: display-time-string-forms

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)'.