Tulvan makes employ of different particles to convey the same meaning as some of our prepositions. For the meaning "in a particular moment" one uses in Tulvan not a postposition but a particle, namely "ni" which has the approximate meaning "in, at" but it's used only for time notions. This means the particle will be used at any time preceding a period of time or a time related word, so, for example when saying goodnight Tulvan says:
trum ni nari
You might remember this expression from before, particularly this post: here. Its actual meaning is "well-ness in the night", remember Tulvan has no proper adjective "good" but uses the adjective affix with the noun. So in this instance "well-ness" or "well-being in the night" means "good night" or at least the salute, and one would never say "itrum nari" which would mean "this is a well-behaved night", or "an useful night", or even "a morally good night".
Interesting to note, however, that Tulvan will use this same particle as classifying one every time a specific moment is mentioned. So for instance:
ni nari gim ni wen cum
from night to day
Direction specifications might be used when the meaning is not clear, but commonly are left out when it's pretty clear the relative position in time or the phrase is self-explanatory. For all this the particle's main meaning might be said as being "in, at".
Kalev vu kwam mem ni oren yulen
I haven't seen you in a thousand years!