Because it is wrong. Time is a difficult problem to solve, it doesn't move at a constant rate, the length of days and years change. You simply cannot set a constant and have it be "correct".

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

Not all months have 30 days.

    T_1_SOLAR_YEAR = 31558150

A solar year, is (roughly) 31556925.25218 seconds[1].

[1]http://www.wolframalpha.com/input/?i=seconds+in+a+solar+year