I am a non-native English speaker. In my language, I could use the expression "moment zero," or "zero moment" to describe the very beginning of a process. I have googled this and it does not seem like something that English speakers would regularly use. (It has a reference to physics, when you describe the zero moment prior to setting a force in motion.) what would be a good equivalent in English? thanks so much for your input!
The word zero is used in several terms of art in various fields. In physics and enginering, usually in the form time t=0. From Advanced Engineering Mathematics by Alan Jeffrey:
At time t = 0 a stretched string of length L, clamped at each end, starts from its equilibrium positionu(x,0) = 0 with the transverse speed ut(x, 0) = ksin(2πx/L). Use simple arguments to find its transverse displacement u(x, t) at any subsequent [time.]
The term moment in physics refers to the vector calculated by force times distance, so "moment zero" might be a distraction in this context.
Zero hour (also known as H hour), as has been noted, is the military jargon for planned start of some activity. For the invasion of Europe on June 6, 1944, this was 0640 hours.
In epidemiological terms, patient zero is the index case, i.e., the first person to come down with the disease in an epidemic.
If you plan to use your term in a specific and technical sense, then pairing a word with zero might be useful. Otherwise, you might be better off with a synonym for beginning.
Sometimes we use the term square one, but usually only when returning to the start of a process or the ground state ('back to square one').