Some history: I appreciate that the timestamp "0 seconds ago" (or "in 0 seconds" in rare cases) has been replaced with "just now" on 3 September 2011. Now, as of 14 September it's "momentarily" for very close future dates and "just now" for very close past dates.
The rationale behind the first change was that "asked in 0 seconds" looks rather awkward (and "just now" also looks more friendly than "0 seconds ago" to me). The rationale behind the second change: "just now" doesn't make sense in "You can accept an answer just now" if it's still 3 seconds to go.
However, the second change broke what was intended with the first one – now we get timestamps like "asked momentarily" (due to clock skew issues, as Jeff Atwood explained. This isn't any better than "asked in 0 seconds".
How to fix: As hammar suggested in a comment, for small time differences, "just now" should always be displayed for things that have to lie in the past, like posting of questions / answers / comments and edits. Similarly, for things that just have to lie in the future, it should always be "momentarily".
(As a side note: Robert Harvey is quite right that "You can accept an answer momentarily" should be avoided entirely if possible; just let the user accept the answer and don't make him wait 3 seconds. Maybe this could apply to all very close future dates?)