date_range_parse.function.php

<?php
function range_parse($date, $default_year) {
	$get_month_num = function($month_name) {
		$month_num = false; //default

		for($i=1; $i <= 12; $i++) {
			$month_start = mktime(12,0, 0, $i, 1);
			$month_long = date('F', $month_start);
			$month_short = date('M', $month_start);
			$month_long = mb_strtolower($month_long);
			$month_short = mb_strtolower($month_short);

			if(
				mb_stripos($month_name, $month_long) === 0 ||
				mb_stripos($month_name, $month_short) === 0
			) {
				//pop off month then try to parse year and day
				$month_num = $i;
				break;
			}
		}

		return $month_num;
	};

	$to_full_year = function($year, $split_point = 65) {
		if(
			$year < 100 &&
			$year >= $split_point
		) {
			$year = "19$year";
		} elseif(
			$year >= 0 &&
			$year < $split_point
		) {
			$year = "20$year";
		}

		return $year;
	};

	$strip_empty = function(array $array){
		$stripped = array_filter($array, function($el) {
			return ($el ==  '') ? false : true;
		});
		$stripped = array_values($stripped);

		return $stripped;
	};

	$date = trim($date);
	$date = mb_strtolower($date);

	$range = new stdClass;

	$month_names = [];
	for($i=1; $i <= 12; $i++) {
		$month_start = mktime(12,0, 0, $i, 1);
		$month_long = date('F', $month_start);
		$month_short = date('M', $month_start);
		$month_names[] = mb_strtolower($month_long);
		$month_names[] = mb_strtolower($month_short);
	}
	$months_or = implode('|', $month_names);

	if( //month
		preg_match("/^($months_or)\s*(,|\s)\s*0*((19|20)[0-9]{2})$/", $date) || // jan ,yyyy
		preg_match("/^($months_or)\s*,\s*0*([0-9]{2})$/", $date) || // jan ,yy
		preg_match("/^($months_or)$/", $date) // jan
	) {
		$date = preg_replace('/(,|\s)/', ' ', $date);
		$date = explode(' ', $date);
		$date= $strip_empty($date);
		$date = array_pad($date, 2, '');
		list($month, $year) = $date;
		$month = ctype_digit($month) ? $month : $get_month_num($month);
		$year = ($year == '') ? $default_year : $year;
		$year = $to_full_year($year);

		$start = mktime(0, 0, 0, $month, 1, $year);
		$days_in_month = date('t', $start);
		$month_length = $days_in_month * 24 * 60 * 60;

		$range->type = 'month';
		$range->formatted = date('F, Y', $start);
		$range->start = $start;
		$range->length = $month_length;
		$range->end = $range->start + $range->length;
	} elseif( //day
		preg_match("/^0*(1[0-2]|[1-9])\/0*(3[0-1]|[1-2][0-9]|[1-9])\/0*((19|20)?[0-9]{2})$/", $date) || // m/d/y
		preg_match("/^($months_or)\s0*(3[0-1]|[1-2][0-9]|[1-9])\s*(,|\s)\s*0*((19|20)?[0-9]{2})$/", $date) || // jan 03, y
		preg_match("/^0*(1[0-2]|[1-9])\/0*(3[0-1]|[1-2][0-9]|[1-9])$/", $date) || // mm/dd
		preg_match("/^($months_or)\s*0*(3[0-1]|[1-2][0-9]|[1-9])$/", $date) // jan dd
	) {
		$date = preg_replace('/(,|\s|\/)/', ' ', $date);
		$date = explode(' ', $date);
		$date = $strip_empty($date);
		list($month, $day, $year) = array_pad($date, 3, '');

		$month = ctype_digit($month) ? $month : $get_month_num($month);
		$year = ($year == '') ? $default_year : $year;
		$year = $to_full_year($year);
		// print_r_pre($year); exit;

		$valid = checkdate($month, $day, $year);
		if($valid) {
			$start = mktime(0, 0, 0, $month, $day, $year);

			$range->type = 'day';
			$range->formatted = date('F j, Y', $start);
			$range->start = $start;
			$range->length = 24 * 60 * 60;
			$range->end = $range->start + $range->length;
		} else {
			$range = false;
		}
	} elseif( //year
		preg_match("/^((19|20)?[0-9]{2})$/", $date) // y
	) {
		$year = date($date);
		$year = $to_full_year($year);

		$start = mktime(0, 0, 0, 1, 1, $year);
		$is_leap_year = date('L', $start);
		$days_in_year = $is_leap_year ? 366 : 365;
		$year_length = $days_in_year * 24 * 60 * 60;

		$range->type = 'year';
		$range->formatted = date('Y', $start);
		$range->start = $start;
		$range->length = $year_length;
		$range->end = $range->start + $range->length;
	} else {
		$range = false;
	}

	return $range;
}
?>

Parses date strings and a time range. Decides whether it's a day, month, or year and returns start time, end time, date type, and the formatted date. returns false for invalid or rejected dates. checks if day of the month is valid too and takes leap years into account. Will overflow and return December 31, 1969 if using 32-bit integers(check PHP_MAX_INT). Useful for running time based queries from a single user input box.

Should be deterministic. If it fails on any input please open an issue asap.

Accepted formats:

Day Formats:
March d, yyyy
March d,yyyy
March d yyyy
March d yy
March d,yy
March d,0000yy
March d,0000yyyy(leading zeros)
m/d/y
m/d
March d

Month Formats;
March
Mar
Mar yy
Mar yyyy

Year Formats:
yyyy
yy(leading zeros not supported here)

Case-insensitive. Full and 3 letter month formats supported. Leading zeros supported except with "yy" year format