Yes, it is UTC. And it says so clearly in the help center/support section.
The US alone has 6 (9) time zones - and the world 24 (25). This is a global platform.
There needs to be some sort of coordination, don't you think? The work week is the same for everybody, no matter where they live.
Work time has been observed in UTC since pretty much the inception of oDesk. There is nothing for Upwork to fix for a change and it's pretty well documented in the support files.
I don't want it to be monday. 6 deadlines soon and for the first time in my freelancing career I had to turn work away because I think I'm overbooked.
Today is a do or die day.
I understand that it's frustrating since it's first time you noticed it. As everybody noted on this thread, the Team App and all the reports are set in UTC by design. You will see a note about it here:
Also, you receive emails notifying you that the Work Week ended and your weekly timelog is available for review. At this point in order for 01:10 to count as your last week's work, you can add it as manual time to last week's logs and remove the screenshots from your Work Diary. Or you can leave is as it is - you will still get paid for the time you logged.
Why should Upwork fix the time if you couldn't be bothered to read the time on your timer?
Talk to you client explain you didn't understand. I am sure the client will allow for manual time if you just explain.