Countdown from a future date and time

[lucianmarin]

There are a few benefits to let users set a future date and time:

• if Mac sleeps then wakes up the countdown clock isn't affected
• they know exactly when a future appointment is
• they can use days, hours, minutes from now to calculate this future date and time in UI
• the standard calendar widget from macOS can be used for this

[nikhilweee]

This would be super helpful!