I see absolutely nothing wrong with this. It replaces a slow way to write human-readable intervals with a fast way. How is that a bad thing?
It may be more performant to set time ranges as constants, but you will get in to trouble unless you know exactly what you are doing. The people at Twitter who wrote the code will be aware of the trade-offs they have made, but what happens when Jo Random developer forks it on GitHub without thinking it through because "it's fast and Twitter use it - it must be good!"
For example:
T_1_MONTH = 2592000
T_1_SOLAR_YEAR = 31558150
[1]http://www.wolframalpha.com/input/?i=seconds+in+a+solar+year
"Difficult for novices to use incorrectly" is a good feature for code to have, but it's not the highest good, and its absence does not mean that the code is bad.
Instead of writing `foo(3.days)` they are writing `foo(T_3_DAYS)` with this library. Instead, why not just create a constant `BAR_DURATION = 3.days.in_seconds` and then write `foo(BAR_DURATION)`.
Problem solved.
It's benign, boring utility code, I just don't get what's offensive about it.