Gets the week number for a given date

gistfile1.dart

main () {

    // get today's date
    var now = new DateTime.now();

    // set it to feb 10th for testing
    //now = now.add(new Duration(days:7));

    int today = now.weekday;

    // ISO week date weeks start on monday
    // so correct the day number
    var dayNr = (today + 6) % 7;

    // ISO 8601 states that week 1 is the week
    // with the first thursday of that year.
    // Set the target date to the thursday in the target week
    var thisMonday = now.subtract(new Duration(days:(dayNr)));
    var thisThursday = thisMonday.add(new Duration(days:3));

    // Set the target to the first thursday of the year
    // First set the target to january first
    var firstThursday = new DateTime(now.year, DateTime.JANUARY, 1);

    if(firstThursday.weekday != (DateTime.THURSDAY))
    {
        firstThursday = new DateTime(now.year, DateTime.JANUARY, 1 + ((4 - firstThursday.weekday) + 7) % 7);
    }

    // The weeknumber is the number of weeks between the
    // first thursday of the year and the thursday in the target week
    var x = thisThursday.millisecondsSinceEpoch - firstThursday.millisecondsSinceEpoch;
    var weekNumber = x.ceil() / 604800000; // 604800000 = 7 * 24 * 3600 * 1000

    print("Todays date: ${now}");
    print("Monday of this week: ${thisMonday}");
    print("Thursday of this week: ${thisThursday}");
    print("First Thursday of this year: ${firstThursday}");
    print("This week is week #${weekNumber.ceil()}");
}

Pay attention that this won't work with daylight-savings.
Edit: The weekNumber is probably correct, but thisMonday or thisThursday could yield bad results.

You can rewrite thisThursday.millisecondsSinceEpoch - firstThursday.millisecondsSinceEpoch using thisThursday.difference(firstThursday)