Calculate which date is x days before/after a date

date-calc-days.js

var MONTHS = {
    1: 31,
    2: 28,
    3: 31,
    4: 30,
    5: 31,
    6: 30,
    7: 31,
    8: 31,
    9: 30,
    10: 31,
    11: 30,
    12: 31
}

var leap = 2020 


// input: 6/6/1999
// input: 66
// input: before 
// output: 4/1/1999


function get_date_from_days (date, days, direction) {
    if (direction === "before") {
        get_date_days_before(date, days)
    } else if (direciton === "after") {
        get_date_days_after(date, days)
    } else {
        throw new Error('direction must be either before or after')
    }
}

function object_from_date_str (date) {
    var arr = date.split("/")
    return { year: Number(arr[2]), month: Number(arr[0]), day: Number(arr[1]) } 
}

function get_date_days_before (date, days) {
    var d = object_from_date_str(date)
    return calculate_days_before(d, days)
}

function calculate_days_before (d, days) {
    var val = loop_calculate_days_before(d, days)
    if (val.complete === true) {
        return val.d
    } else {
        return calculate_days_before(val.d, val.days)
    }
}

function loop_calculate_days_before (d, days) {
    if (d.year % 4 === 0) MONTHS[2] = 29
    else MONTHS[2] = 28
    if (days === 0) return { d: d, days: days, complete: true }
    days = days - 1
    if (d.day > 1) {
        d.day = d.day -1
        return { d:d, days: days, complete: false }
    } else {
        // day of month is 1 
        if (d.month > 1) {
            d.month = d.month - 1
            d.day = MONTHS[d.month]
            return { d:d, days: days, complete: false }
        } else {
            // month is 1 
            d.year = d.year - 1
            d.month = 12
            d.day = MONTHS[d.month]
            return { d:d, days: days, complete: false }
        }
    }

}

function get_date_days_after (date, days) {
    var d = object_from_date_str(date)
    return calculate_days_after(d, days)
}

function calculate_days_after (d, days) {
    var val = loop_calculate_days_after(d, days)
    if (val.complete === true) {
        return val.d
    } else {
        return calculate_days_after(val.d, val.days)
    }
}

function loop_calculate_days_after (d, days) {
    if (d.year % 4 === 0) MONTHS[2] = 29
    else MONTHS[2] = 28
    if (days === 0) return { d: d, days: days, complete: true }
    days = days - 1
    if (d.day < MONTHS[d.month]) {
        d.day = d.day + 1
        return { d:d, days: days, complete: false }
    } else {
        // day of month is at the end 
        if (d.month < 12) {
            d.month = d.month + 1
            d.day = 1
            return { d:d, days: days, complete: false }
        } else {
            // month is 12 
            d.year = d.year + 1
            d.month = 1
            d.day = 1
            return { d:d, days: days, complete: false }
        }
    }

}

get_date_days_before("6/6/1999", 66)
get_date_days_after("6/6/1999", 66)
get_date_days_before("6/6/1999", 666)
get_date_days_after("6/6/1999", 666)




var days_arr = [
    11,
    22,
    33,
    44,
    55,
    66,
    77,
    88,
    99,
    111,
    222,
    333,
    444,
    555,
    666,
    777,
    888,
    999,
    420,
    707,
    712,
    717,
    725,
    1111,
    2222,
    3333,
    4444,
    5555,
    6666,
    7777,
    8888,
    9999
]


function calculate_all (date) {
    days_arr.forEach(function (day) {
        console.log(day, "before", get_date_days_before(date, day))
        console.log(day, "after", get_date_days_after(date, day))
    })
}

calculate_all("9/11/2001")
calculate_all("6/6/2020")