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var date = moment();
var currentMonth = date.month();

var daysInMonth = 31;         // Every month has 31 days
if (currentMonth % 2 == 1) {  // except every other month
    if (currentMonth == 1) {  // and then there is februrary
        if (date.isLeapYear()) { 
            daysInMonth = 29; // which has 29 if it's a leap year
        } else {
            daysInMonth = 28; // and 28 if it isn't.
        }
    } else {
        daysInMonth = 30;
    }
}