When I hover over the triangle on StackExchange, I find I have gained 45 rep today, but only 30 this week. Is there a different cutoff time?
It's not a different cut-off time, but rather how we calculate it, for "today" we count up the votes that happened today, where as with a week we track the net change throughout the week.
I'm going to look at changing this tomorrow (so that the day does the same as other intervals). There are pros/cons on both solutions, but the consistency might be worth it here.
Here's the common case that leads to this: A vote is deleted or undeleted that was previously created, this makes display a bit wacky since we don't show removed votes, or undeleted votes in the time they had the net effect, let's take an example:
- May: someone votes your post up, +10
- your rep goes up 10 for the week, month, etc.
- June: say you edited later, someone removes that vote (or unaccepts an answer)
- your rep goes down 5/10/15, etc for the week, month, etc.
Now from the day point of view (strictly looking at that range), your rep hasn't changed, no vote happened then. However, there was a net effect on your rep, it's just that the 10 rep was removed (looking overall) back in may...or rather it no longer happened.
In your particular case, this answer was unaccepted, resulting in a -15 net when it happened today.