Yesterday, there was a brief power outage in Xiao Yu's neighborhood. His digital alarm clock reset and lost some time. When the power came back on, the clock started running again, but Xiao Yu forgot to re-sync it with the standard time. The next morning, a time puzzle was born.

• His clock woke him up at 7:10 AM.
• He arrived at school just as the big clock at the gate showed 7:50 AM, 10 minutes before class started.
• School was dismissed at 5:00 PM.
• When he got home, his own clock showed 4:40 PM.

Assuming his travel time to and from school is identical, can you figure out exactly how long his clock was stopped?