Date . prototype . addBusinessDays = function ( businessDays : number ) { // 5 business days in a week const weeks = Math . floor ( businessDays / 5) ; // Convert business days (5 days in a week) to regular days (7 days in a week) const days = weeks * 7 ; const date = moment ( this ) ; // Convert the incoming date value to a moment value. const newDate = moment ( this ) . add ( days + ( businessDays % 5) , 'days' ) ; if ( newDate . day () < date . day () || newDate . day () % 6 === 0) { // Add 2 more days if we land on a weekend, or we went through a weekend. newDate . add (2 , 'days' ) ; // .add is not pure and will change the underlying value } return newDate . toDate () ; } ;