The time changed. You changed your clocks. Did you change the battery in smoke alarm?
Also important, how old is your smoke alarm? They need to be replaced after 10 year. If the manufacturing date (on the back) is 10 or more years ago, or it has a date code or no indication at all, REPLACE IT!
I am a fire investigator. I have investigated a lot of fatal fires, but never one where there was one or more working smoke alarms in the home.