Date Arithmetic
When I set an alarm, my clock, now running on the computer in my pocket, is smart enough to tell me how much time will pass until the alarm is scheduled to sound. This has eliminated the old problem of sleeping past meetings before being surprised by an alarm precisely half a day after I had originally planned to wake.
The price has been having to know exactly how little I will sleep: a usually depressing fact that had previously been obscured by my difficulty doing time arithmetic in my most somnolent moments.