It is customary to say, simply, "point in time" — as both "timepoint" and "time point," while understandable, are not used much.
If you're talking about programming exclusively, then the question is probably too narrowly scoped for this SE site. Whatever your company uses is what you should use. Note that style checkers probably looked up "timepoint" and didn't find it, but the two-word confection of "time point" renders two very basic words that check out just fine.
Although the Google NGram viewer has its flaws, in this case you may notice that "timepoint" and "time point" gained what currency they have as computers came into wide usage. One may infer from this that the technical aspect is the reason, which supports the contention that you should use whatever your audience is most comfortable with. Since "point in time" still leads by a comfortable margin, it is the term you should favor for non-technical uses.
Disclaimer: There is a usage of "time point" in music, referring to the start of a tone in a tone row. This usage, coined by Milton Babbitt in 1962, probably adds something extra to the "time point" curve.