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HTML 

              
                - const startingYear = 2010;
- const endingYear   = 2020;
- const monthsCount  = 12;
- const datePattern  = 'd M Y';
- const daysInWeek   = 7;
- const currentDate  = new Date();

//- 'id' and 'name' must be changeable (by form; for multiple datepicker)
- const idPrefix = 'cal_';
- const calName  = 'calendar';
- const dateName = 'calendar_date';
			
//- TODO: Make variable:'daysLabel' changeable by language
- const daysLabel = [
- 'Sun',
- 'Mon',
- 'Tue',
- 'Wed',
- 'Thu',
- 'Fri',
- 'Sat'
- ];
//- TODO: Make variable:'monthsLabel' changeable by language
- const monthsLabel = [
- 	'January',
- 	'February',
- 	'March',
- 	'April',
- 	'May',
- 	'June',
- 	'July',
- 	'August',
- 	'September',
- 	'October',
- 	'November',
- 	'December'
- ];

.calendar
	- for (var year = startingYear; year <= endingYear; year++)
		- for (var month = 0; month < monthsCount; month++)
			//- For the year/month picker
			input(
				type="radio"
				id=idPrefix + "year_select_" + year + "_" + month
				name=calName
			).cal-year-check
			input(
				type="radio"
				id=idPrefix + "month_select_" + year + "_" + month
				name=calName
				data-cal-year=year
			).cal-month-check
			.cal
				.cal-head
					.controls
						label(for=idPrefix + year + "_" + month).cal-change-year #{monthsLabel[month]} #{year}
							// Borrowed Google's 'Material Design Icons'
							svg(viewBox="0 0 24 24")
								path(d="M13,20H11V8L5.5,13.5L4.08,12.08L12,4.16L19.92,12.08L18.5,13.5L13,8V20Z")
					.controls
				.cal-body.year-picker
					- for (var yearPick = startingYear; yearPick <= endingYear; yearPick++)
						.cal-date
							label(for=idPrefix + "month_select_" + yearPick + "_" + month)
								span #{yearPick}
				- for (var yearPick = startingYear; yearPick <= endingYear; yearPick++)
					.cal-body.month-picker(data-cal-year=yearPick)
						- for (var monthPick = 0; monthPick < monthsCount; monthPick++)
							.cal-date
								label(for=idPrefix + yearPick + "_" + monthPick)
									span #{monthsLabel[monthPick]}
			//- For the actual calendar
			input(
				type="radio"
				id=idPrefix + year + "_" + month
				name=calName
				checked=currentDate.getFullYear() == year && currentDate.getMonth() == month
			).cal-check
			.cal
				.cal-head
					.controls
						label(for=idPrefix + "year_select_" + year + "_" + month).cal-change-year #{monthsLabel[month]} #{year}
							// Borrowed Google's 'Material Design Icons'
							svg(viewBox="0 0 24 24")
								path(d="M11,4H13V16L18.5,10.5L19.92,11.92L12,19.84L4.08,11.92L5.5,10.5L11,16V4Z")
					.controls
						- var lastMonth = new Date(year, month - 1);
						- var nextMonth = new Date(year, month + 1);
						label(for=idPrefix + lastMonth.getFullYear() + "_" + lastMonth.getMonth()).cal-change-month
							// Borrowed Google's 'Material Design Icons'
							svg(viewBox="0 0 24 24")
								path(d="M20,11V13H8L13.5,18.5L12.08,19.92L4.16,12L12.08,4.08L13.5,5.5L8,11H20Z")
						label(for=idPrefix + nextMonth.getFullYear() + "_" + nextMonth.getMonth()).cal-change-month
							// Borrowed Google's 'Material Design Icons'
							svg(viewBox="0 0 24 24")
								path(d="M4,11V13H16L10.5,18.5L11.92,19.92L19.84,12L11.92,4.08L10.5,5.5L16,11H4Z")
				.cal-body
					- var firstDate = new Date(year, month, 1);
					- var lastDate  = new Date(year, (month + 1), 0);
					//- Days of week
					- for (var day = 0; day < daysInWeek; day++)
						.cal-day #{daysLabel[day]}
					//- Extra spaces before starting date
					- for (var date = 0; date < firstDate.getDay(); date++)
						.cal-date
					//- Actual date
					- for (var date = 1; date <= lastDate.getDate(); date++)
						//- TODO: Date pattern must be changeable
						.cal-date
							//- Name here must be changeable
							label
								input(
									type="radio"
									name=dateName
									value=year + "/" + month + "/" + date
									checked=currentDate.getFullYear() == year && currentDate.getMonth() == month && currentDate.getDate() == date
								)
								span #{date}
					//- Extra spaces after ending date
					- for (var date = lastDate.getDay() + 1; date < daysInWeek; date++)
						.cal-date
!

CSS 

              
                // ----- About: Calendar
$starting-year: 2010;
$ending-year: 2020;
$months-count: 12;
$days-in-week: 7;

// ----- Global
$cell-size: 100%;

$hover-background-color: #efefef;

$primary-color: #ffffff;
$primary-background-color: #0681ff;

// ----- Styles
$calendar-background-color: #ffffff;
$calendar-width: 280px;
$calendar-box-shadow: 0 5px 10px 2px rgba(0, 0, 0, 0.5);

$cal-head-color: $primary-color;
$cal-head-background-color: $primary-background-color;
$cal-head-height: 50px;

$cal-head-controls-width: 30px;
$cal-head-controls-height: $cal-head-height;

$cal-change-month-width: $cal-head-controls-width;
$cal-change-month-height: $cal-head-controls-height;

$cal-change-month-active-background-color: darken($primary-background-color, 10%);

$cal-change-month-svg-fill: #ffffff;
$cal-change-month-svg-width: 24px;
$cal-change-month-svg-height: 24px;

$cal-change-year-height: $cal-head-controls-height;
$cal-change-year-padding: 0 8px;

$cal-change-year-svg-width: 12px;
$cal-change-year-svg-height: 12px;
$cal-change-year-svg-fill: #ffffff;

$cal-change-year-active-background-color: darken($primary-background-color, 10%);

$cal-day-background-color: #dfdfdf;
$cal-day-height: 30px;

$cal-date-height: 40px;

$cal-date-span-hover-background-color: $hover-background-color;
$cal-date-span-active-color: $primary-color;
$cal-date-span-active-background-color: $primary-background-color;
$cal-date-span-transition: background-color ease-out 150ms, color ease-out 150ms;

$cal-body-min-height: $cal-day-height + ($cal-date-height * 6);

// ----- SCSS codes
.calendar {
	background-color: $calendar-background-color;
	width: $calendar-width;
	margin: auto;
	user-select: none;
	box-shadow: $calendar-box-shadow;
	flex-grow: 0;
	flex-shrink: 0;
	.cal-check {
		display: none;
		&:checked + .cal {
			display: block;
		}
	}
	.cal-year-check {
		display: none;
		&:checked + .cal-month-check + .cal {
			display: block;
		}
	}
	@for $year from $starting-year through $ending-year {
		.cal-month-check[data-cal-year="#{$year}"] {
			display: none;
			&:checked + .cal {
				display: block;
				.year-picker {
					display: none;
				}
				.month-picker[data-cal-year="#{$year}"] {
					display: grid;
				}
			}
		}
	}
	.cal {
		display: none;
		.cal-head {
			color: $cal-head-color;
			background-color: $cal-head-background-color;
			width: 100%;
			height: $cal-head-height;
			display: flex;
			.controls {
				width: 50%;
				height: $cal-head-controls-height;
				line-height: $cal-head-controls-height;
				box-sizing: border-box;
				&:first-child {
					font-weight: bold;
					text-align: left;
				}
				&:last-child {
					display: flex;
					justify-content: flex-end;
				}
				.cal-change-year {
					height: $cal-change-year-height;
					line-height: $cal-change-year-height;
					padding: $cal-change-year-padding;
					display: inline-block;
					&:hover, &:active {
						background-color: $cal-change-year-active-background-color;
					}
					svg {
						width: $cal-change-year-svg-width;
						height: $cal-change-year-svg-height;
						fill: $cal-change-year-svg-fill;
						vertical-align: middle;
					}
				}
				.cal-change-month {
					text-align: center;
					width: $cal-change-month-width;
					height: $cal-change-month-height;
					line-height: $cal-change-month-height;
					display: flex;
					&:hover, &:active {
						background-color: $cal-change-month-active-background-color;
					}
					svg {
						width: $cal-change-month-svg-width;
						height: $cal-change-month-svg-height;
						fill: $cal-change-month-svg-fill;
						vertical-align: middle;
						margin: auto;
					}
				}
			}
		}
		.cal-body {
			min-height: $cal-body-min-height;
			display: grid;
			align-content: start;
			grid-template-columns: repeat(7, 1fr);
			&.year-picker {
				grid-template-columns: repeat(4, 1fr);
			}
			&.month-picker {
				display: none;
				grid-template-columns: repeat(3, 1fr);
			}
			.cal-day {
				background-color: $cal-day-background-color;
				height: $cal-day-height;
				line-height: $cal-day-height;
				text-align: center;
			}
			.cal-date {
				height: $cal-date-height;
				line-height: $cal-date-height;
				text-align: center;
				input[type="radio"] {
					display: none;
					&:checked + span {
						color: $cal-date-span-active-color;
						background-color: $cal-date-span-active-background-color;
					}
				}
				label, label span {
					text-align: center;
					display: block;
				}
				label span {
					transition: $cal-date-span-transition;
					&:hover {
						background-color: $cal-date-span-hover-background-color;
					}
				}
			}
		}
	}
}

// -----

html, body {
	width: 100%;
	height: 100%;
	padding: 0;
	margin: 0;
}

@import url("https://fonts.googleapis.com/css?family=Montserrat:400,400i,700");

body {
	background-color: #082946;
	display: flex;
	.calendar {
		font-family: Montserrat, sans-serif;
		font-size: 14px;
	}
}
!

JS
!
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