I wanted to write
1 Monat ± 1 Tag
in German (which still isn't). Then I noticed I didn't even know how to write this with
-. It seems, or I feel, like I need to decline ein Tag.
Indeed minus is a preposition which accepts everything as case (except the boring nominative, which nobody expected):
minus Präposition mit Dativ, Akkusativ oder Genitiv: vermindert um, verringert um
So, should I assume plus obeys the same rule?
Finally, would you have objections if I go with dative for plus/minus? This being the right case, just for completeness, I would write
Ein Monat plus/minus einem Tag wäre die Dauer. (?)