TypeScript date helper functions

DateHelper.ts

export class DateHelper {
    public static getWeekDay(locale: string, day: number, format: string = 'long') {
        if (day <=0 || day > 7) throw new Error('Invalid day number. Expected to be in range of 1-7.');
        let baseDate = new Date(Date.UTC(2000, 0, 3)); // Monday
        baseDate.setDate(baseDate.getDate() + day - 1);
        return baseDate.toLocaleDateString(locale, { weekday: format });
    }

    public static getMonth(locale: string, month: number, format: string = 'long') {
        if (month <=0 || month > 12) throw new Error('Invalid month number. Expected to be in range of 1-12.');
        let baseDate = new Date(Date.UTC(2000, month - 1, 1)); // January
        return baseDate.toLocaleDateString(locale, { month: format });
    }

    public static getAdjacentMonth(date: Date, when: string) : Date {
        var month = date.getMonth();
        switch (when) {
            default: throw new Error("Invalid when parameter. Expected to be 'before' or 'after'.");
            case 'before': month -= 1; break;
            case 'after': month += 1; break;
        }
        return new Date(new Date(date.setMonth(month)).setDate(1))
    }

    public static getDayOfMonth(date: Date, what: string) : Date {
        switch (what) {
            default: throw new Error("Invalid what parameter. Expected to be 'first' or 'last'.");
            case 'first': return new Date(date.getFullYear(), date.getMonth(), 1);
            case 'last': return new Date(date.getFullYear(), date.getMonth() + 1, 0);
        }
    }
    
    public static getDaysInMonth (date: Date) {
       return new Date(date.getDay(), date.getMonth(), 0).getDate();
    }

    public static addDays(date: Date, value: number) : Date {
        return new Date(date.getFullYear(), date.getMonth(), date.getDate() + value);
    }

    public static resetHours(date: Date, when: string) : Date {
        switch (when) {
            default: throw new Error("Invalid when parameter. Expected to be 'start' or 'end'.");
            case 'start': return new Date(date.setHours(0, 0, 0, 0));
            case 'end':  return new Date(date.setHours(23, 59, 59, 59));
        }
    }
}