dateDiffInDays method
Implementation
static double dateDiffInDays(DateTime a, DateTime b) {
// adapted from http://stackoverflow.com/a/15289883/251162
const MS_PER_DAY = 1000 * 60 * 60 * 24;
// Disstate the time and time-zone information.
var utc1 = DateTime.utc(a.year, a.month, a.day);
var utc2 = DateTime.utc(b.year, b.month, b.day);
return (utc2.difference(utc1)).inMilliseconds / MS_PER_DAY;
}