There's a few reasons we don't do other timezones on the main sites. In this case, it's not technically feasible...nor would it be fair.
Your week reputation is compared to others in the weekly tab of the
/users page, so that needs to be consistent (reset at the same time). Your week resets at the exact same time as everyone else for the reputation leagues.
Also, showing a different date range for the dropdown isn't something we'd want to do, since it matches no other "week" in the system (note the stackexchange.com leagues use the same reset-on-sunday week).
We stick with a consistent UTC 00:00 reset every day for many things, time is an immensely complicated thing that also changes frequently. Let's take two quick examples:
- Today was daylight saving's time change in the US, should we reset the week twice when time moves?
- Timezones vary per country, some of them change DST on a whim.
As simple as time seems to be, it's not. To avoid confusion there is one way: consistency. The date/time shown on all posts, etc is always UTC time, and that's what we also use for reputation intervals.
Somewhat related: from a descriptive standpoint, a "week" needs to be a consistent term, all reputation categories can be described as:
"From the stat of the X"
X being "week", "month", "quarter", "year"...and "all time" is pretty intuitive as well (though it doesn't exactly fit the above phrase..."beginning of time" was too long, sorry!)