Apparently, there is a slight difference, but only in British English: Note: In British English, 'onwards' is an adverb and 'onward' is an adjective. In American English and sometimes in formal British English, 'onward' may also be an adverb. So, depending on where you are, the difference maybe slight. However, they both mean the same thing practically, and most people will understand you if ...
Onward sounds odd and is uncommon at the start of a sentence. Here's the definition of onward in the Oxford English Dictionary, adverb, entry 4b. In an uninterrupted advance or succession forward in time. Cf. on adv. 4a. The definition of on referred to is the same on used in the adverbial phrase "from now on." So the two have some relation to one another. My issue with your example is that ...
adverbs - Onward at the beginning of a sentence - English Language ...
It's not unheard of for onwards (or usually, onward --see also backward vs. backwards) to be used as an interjection, so there's nothing wrong with using it as a single-word command, like "Go!" It's a bit overwrought, though, so I wouldn't recommend using it in formal settings unless you're being intentionally humorous or colloquial.
Can Onwards be used as a sentence? - English Language & Usage Stack ...
"From this time forward" isn't really used in colloquial English. Even henceforth is somewhat archaic. People frown upon it as American English but "going forward" is becoming popular. "From now on" would probably be the most common phrase used to mean henceforth. Regarding the origin - hence in this context means 'from now' or 'from here' and forth means onward, i.e. from now onwards or from ...