- Visit https://en.wikipedia.beta.wmflabs.org/wiki/Special:Preferences?ooui=1#mw-prefsection-rendering-timeoffset
- Choose "Other (specify offset)" in timezone (if JS is enabled else this is not required).
The empty textbox is not so helpful as it doesn't specify the format of the offset. Further when JS is disabled, even it's purpose is not clear.
The textbox could use the placeholder attribute to specify information about it's purpose and format of the offset.