{{(moment(dateRange.value.end).valueOf() - moment(dateRange.value.start).valueOf()) / 1000 / 60 / 60 / 24}}
It's that easy!
valueOf()
returns the millisecond of the moment in the Unix epoch which started 00:00:00 UTC on 1 January 1970. The consecutive divisions take us down to whole days (I've left them all there for clarity rather than just dividing by 86,400,000).
That's all, just thought it was time to give back...