Date Difference Calculation Fix

$('#days_cnt').datepicker().on('changeDate', function(ev) {
//alert("startdate");
var start_date_time_tt = $("#start_date_time").val();
var startdate = new Date(start_date_time_tt);
var enddate = new Date(ev.date);

var day_diff = Date.daysBetween(startdate,enddate);
var report_days_val = $("#reminder_participant").val();

if(report_days_val==''){
var report_days_new = day_diff;
}else{
var report_days_new = report_days_val+','+day_diff;
}
$("#reminder_participant").val(report_days_new);
});
above not working

### Updated Code:

<a id='days_cnt' style="padding-left: 50px; cursor: pointer;">Calculate Days</a>
<input type="text" id="start_date_time" placeholder="Start Date (yyyy-mm-dd)">
<input type="text" id="reminder_participant" placeholder="Reminder Days">

// Initialize the datepicker
$('#days_cnt').datepicker({
    format: "yyyy-mm-dd",
    startDate: '-0d',
    autoclose: true
});

// Calculate days between two dates
$('#days_cnt').datepicker().on('changeDate', function(ev) {
    var start_date_time_tt = $("#start_date_time").val();
    if (!start_date_time_tt) {
        alert("Please select a start date first.");
        return;
    }

    var startdate = new Date(start_date_time_tt);
    var enddate = new Date(ev.date);

    if (isNaN(startdate.getTime()) || isNaN(enddate.getTime())) {
        alert("Invalid dates. Please ensure both dates are in the correct format.");
        return;
    }

    // Calculate day difference
    var day_diff = Math.ceil((enddate - startdate) / (1000 * 60 * 60 * 24));

    if (day_diff < 0) {
        alert("End date cannot be before the start date.");
        return;
    }

    // Update the reminder participant value
    var report_days_val = $("#reminder_participant").val();
    var report_days_new = report_days_val ? report_days_val + ',' + day_diff : day_diff;

    $("#reminder_participant").val(report_days_new);
});

### Key Fixes:
1. **`Date.daysBetween` Issue**: JavaScript does not have a built-in `daysBetween` method. Instead, the difference is calculated using `enddate – startdate` in milliseconds, and converted to days using the formula:

(enddate - startdate) / (1000 * 60 * 60 * 24)

2. **Validation**: Added validation to check if the `start_date_time` input is empty or invalid before proceeding.
3. **Datepicker Element**: You initialized the datepicker on `#days_cnt`, but it's typically used on an input field. Ensure your use case aligns with this.
4. **Negative Differences**: Checked if the calculated day difference is negative and added an error message if `enddate` is before `startdate`.

### Usage Instructions:
1. Enter a start date in the `#start_date_time` field.
2. Click on the `#days_cnt` link and pick a date using the datepicker.
3. The difference in days will be calculated and added to the `#reminder_participant` field.