Converting C# date-time function to Javascript

Question

I have an application in which I use a couple of date/time manipulation function to populate a couple of calendars. Basically, a user selects a month/year from a dropdown (say, March 2019) and I populate the calendars with 03/01/2019 and 03/31/2019.

I wanted to do this client side so tried to convert those function to javascript and I am getting strange results and can't see what I am doing wrong.

This is the original C# functions I defined and used:

public static DateTime FirstDayOfMonth(this DateTime dt)
{
    return new DateTime(dt.Year, dt.Month, 1);
}

public static DateTime LastDayOfMonth(this DateTime dt)
{
    DateTime dtFirstDayOfMonth = new DateTime(dt.Year, dt.Month, 1);
    DateTime dtLastDayOfMonth = dtFirstDayOfMonth.AddMonths(1).AddDays(-1);
    return dtLastDayOfMonth;
}

I called these like below:

DateTime dtSelected = DateTime.Today.AddMonths(int.Parse(ddlMonth.SelectedValue)).AddYears(-1);
dtStartDate = Utils.FirstDayOfMonth(dtSelected);
dtEndDate = Utils.LastDayOfMonth(dtSelected);

The dropdown list is populated like:

for (int i = 12; i >= 1; i--)
{
    string s = DateTime.Now.AddYears(-1).AddMonths(i).ToString("Y");
    ListItem li = new ListItem(s, i.ToString());
    ddlMonth.Items.Add(li);
}

The dropdown entries would look like:

May, 2019    -- value of 12
April, 2019  -- value of 11
....
July, 2018   -- value of 2
June, 2018   -- value of 1

This is my attempt at translating to javacript:

function firstDayOfMonth(dt) {debugger
    var year = dt.getFullYear();
    var month = dt.getMonth();
    var day = dt.getDate();
    return new Date(year, month, 1);
}

function lastDayOfMonth(dt) {debugger
    var year = dt.getFullYear();
    var month = dt.getMonth();
    var day = dt.getDate();
    var firstDayOfMonth = new Date(year, month, 1);
    var lastDayOfMonth = firstDayOfMonth.AddMonths(1).AddDays(-1); --> shows error when called; Object doesn't support property or method 'AddMonths'

    return lastDayOfMonth;
}

$(document).on('change', '#ddlMonth', function () {debugger
    var monthID = this.value;
    var ddlMonth = $('#ddlMonth');
    var today = new Date();
    var startDate = new Date();
    var endDate = new Date();
    var dtSelected = new Date();

    if (ddlMonth.val() == "")
    {
        ....
    }
    else
    {debugger
        dtSelected.setMonth(dtSelected.getMonth() + ddlMonth.val() + 1); -- this becomes "Wed Oct 12, 2360" if I select "March, 2019" from dropdown!
        dtSelected.setFullYear(dtSelected.getFullYear() - 1);
        dtStartDate = firstDayOfMonth(dtSelected);
        dtEndDate = lastDayOfMonth(dtSelected);
    }

Answer 1

you have some problem in your code, this for example:

dtSelected.setMonth(dtSelected.getMonth() + ddlMonth.val() + 1); -- this becomes August of 2036!

you take today's month which is the 5 then add to itsome value from your slect and add 1 more and this is a lot of monthes to add.

I think you need to changesome things and do this like this: change the select values to be the date of the first day of each month and not just number:

for (int i = 12; i >= 1; i--)
{
    DateTime date = DateTime.Now.AddYears(-1).AddMonths(i);
    ListItem li = new ListItem(date.ToString("Y"), date.ToString("yyyy-MM-01"));
    ddlMonth.Items.Add(li);
}

in this format you can use it on js with no problem.

now for your js function:

function lastDayOfMonth(dt) {debugger
    var year = dt.getFullYear();
    var month = dt.getMonth();
    var day = dt.getDate();
    var lastDayOfMonth = new Date(year, month, 1);
    lastDayOfMonth.setMonth(lastDayOfMonth.getMonth() + 1);
    lastDayOfMonth.setDate(lastDayOfMonth.getDate() - 1);

    return lastDayOfMonth;
}

$(document).on('change', '#ddlMonth', function () {debugger
    var ddlMonth = $('#ddlMonth');
    dtStartDate = new Date(ddlMonth.val());
    dtEndDate = lastDayOfMonth(dtStartDate);
 }