interpolateDate function
Creates a date interpolator between a and b.
Implementation
Interpolator<DateTime> interpolateDate(DateTime a, DateTime b) {
final aMs = a.millisecondsSinceEpoch;
final bMs = b.millisecondsSinceEpoch;
return (double t) =>
DateTime.fromMillisecondsSinceEpoch((aMs + (bMs - aMs) * t).round());
}