Use a variable PHP datestring to format a date in Javascript

flatpickr php date.js

/**
 * Localize a Datepicker / Timepicker (flatpickr)
 * Format PHP datestring to js and vice versa
 *
 * fromPhp is the variable that gets passed via wordpress wp_localize_script
 * fromPhp.dateFormat  get_option( 'date_format' );
 * fromPhp.timeFormat	get_option( 'time_format' );
 */

//https://github.com/kartik-v/php-date-formatter
var dateFormatter = new DateFormatter(); //php to js date format


//https://flatpickr.js.org/examples/#datetime
var picker = $('#datepicker').flatpickr({
	altInput: true,
	enableTime: true,
	time_24hr: fromPhp.twentyfour, //24h vs. AM/PM
	defaultHour: 00, //default is 12 but we use 00:00 as "all day"
	defaultMinute: 00,
	parseDate: function(date) {
		return dateFormatter.parseDate(date, 'Y-m-d H:i:s'); //this is the default format we use in the database
	},
	onChange: function(selectedDates, datestr, instance){
		dateObj = selectedDates[0]; //select first of the selected dates
		var phpFormat = dateFormatter.formatDate(dateObj, 'Y-m-d H:i:s');
		/* the fomatDate method formats both the real and the altInput field.
        * So we formate the real Input field again to be compatible with php
        * Might be easier to just use a unix date... */
		picker.input.value = phpFormat; //this

	},
	formatDate: function(dateObj, formatString) {
		var dateTime = fromPhp.dateFormat+' '+fromPhp.timeFormat; //get the php date format
		return dateFormatter.formatDate(dateObj, dateTime); //use the php date formate to show the date object
	}
});