Number of Days Between Two Dates
<p>Write a program to count the number of days between two dates.</p>

<p>The two dates are given as strings, their format is <code>YYYY-MM-DD</code>&nbsp;as shown in the examples.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<pre><strong>Input:</strong> date1 = "2019-06-29", date2 = "2019-06-30"
<strong>Output:</strong> 1
</pre><p><strong class="example">Example 2:</strong></p>
<pre><strong>Input:</strong> date1 = "2020-01-15", date2 = "2019-12-31"
<strong>Output:</strong> 15
</pre>
<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li>The given dates are valid&nbsp;dates between the years <code>1971</code> and <code>2100</code>.</li>
</ul>
Math; String
function daysBetweenDates(date1: string, date2: string): number {
    return Math.abs(calcDays(date1) - calcDays(date2));
}

function isLeapYear(year: number): boolean {
    return year % 4 === 0 && (year % 100 !== 0 || year % 400 === 0);
}

function daysOfMonth(year: number, month: number): number {
    const days = [31, isLeapYear(year) ? 29 : 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31];
    return days[month - 1];
}

function calcDays(date: string): number {
    let days = 0;
    const [year, month, day] = date.split('-').map(Number);
    for (let y = 1971; y < year; ++y) {
        days += isLeapYear(y) ? 366 : 365;
    }
    for (let m = 1; m < month; ++m) {
        days += daysOfMonth(year, m);
    }
    days += day - 1;
    return days;
}
