In the comments of an answer on English Language Learners, I explained to a non-native speaker the usage of "tonight" as being something like "the current or forthcoming night": during the nighttime, it refers to the current night; during the day it refers to the night that will start at the end of that day.
Another user disagrees, and says:
Tonight is an ambiguous word. Generally, I have to clarify with those around me whether they're talking about "now" or the night period that just elapsed (if it's morning) or talking about the next night period that has not yet begun. Most people don't seem to realize there was a need to disambiguate their usage of the term.
The emphasis is mine, and highlights a usage of "tonight" which I don't believe I've ever encountered.
I'm a native (UK) English speaker and wrote my explanation primarily based on my own experience and intuition, but I did check several dictionaries (both UK and US) before posting it to make sure the advice wasn't misguided.
For example, Oxford Dictionaries provides the following definition:
On the present or approaching evening or night.
on this present night or the night following this present day
(during) the night of the present day:
On or during the present or coming night.
All of the above definitions correspond to my understanding of the word, and seem to me to unambiguously exclude the interpretation as "the night period that just elapsed". I did find references to that interpretation on dictionary.com:
Obsolete. during last night.
and at Collins:
obsolete
last night
but in both cases the definition is marked as obsolete, which I take to mean it is no longer in common usage (or if it is, the usage is confined to one or more relatively small areas).
I'd be very interested if anybody could provide evidence (preferably not just anecdotal) for or against this usage being widespread in English today (and any geographical trends relating to the usage). I asked the other user about their location and they said they
live in the USA and have lived in various places throughout the country. In every single locale, I've dealt with ambiguous usage of "tonight"
so this wouldn't be explained by a usage local to some small area.
Best Answer
The relevant definition in the full (subscription-only) OED is...
...where the dagger symbol † indicates "obsolete" (as also explicitly mentioned in the definition).
As an indication of just how obsolete that sense is, it's worth noting that the most recent citation for that sense is...
...so it was obviously considered dialectal/non-standard/antiquated over two centuries ago (back when yesternight was still thought of as a "normal" word). There's no mention on the relevant OED page of any (past or present) US usage difference, so I doubt there's any significance to that aspect.