The circuit components are rated to the industrial temperature range of -40C to +85C, but I think at extreme temperatures you’re more likely to encounter problems with your battery rather than the Mayfly circuit. LiPo batteries don’t perform well at either really cold or really hot temperatures, so you might consider a different type of battery chemistry if you’re deploying it long-term in a harsh environment.
Last winter, a number of my Mayfly loggers saw -20 degrees on a number of occasions according to the logged temperature of the onboard DS3231.