I have had some trouble interpreting the {obl,npmod}:tmod as currently implemented in EWT. If :tmod is part of the deprel, I would have assumed it was to distinguish certain constructions that have special syntax because they are temporal (such as prepositionless oblique nominal modifiers), basically as a temporal counterpart to the :npmod relations. However, this is not a universal opinion:

Originally posted by @dan-zeman in #508 (comment):

I don't see why preposition should make a difference. If you see a pattern in data, it does not necessarily mean that the pattern is a rule. It is also possible that there is an annotation error.

The English documentation has not been updated to UD v2 and the relevant page was named nmod-tmod until now. I just renamed the page and replaced nmod with obl. The second example clearly shows that preposition can occur in obl:tmod: http://universaldependencies.org/en/dep/obl-tmod.html

The guidelines quoted above make it sound like purely a semantic class distinction. Why, then, does it belong in a deprel?

There are in-between cases like measure phrases which appear in the :npmod docs but can be temporal ("5 miles longer/away" vs. "5 years later/ago", "10 dollars an hour" vs. "once a month").

Section 7 of the Mischievous Nominal Constructions paper discussed :npmod and :tmod and argued that both should be limited to adverbial (prepositionless) nominals. A different approach that might be simpler to interpret would be to limit :tmod to certain temporally specific constructions like dates.