As Keystone already uses moment.js, I would like to propose a DateRange field Type using the moment plugin moment-range as mentioned on the moment docs.

The UI could display this as either two date pickers or (my preference) a single date picker which treats the first click as the start and a second click as the end - or drag, similar to many sites especially travel related.

Source: https://github.com/keystonejs/keystone/issues/232