JavaScript/Date calculator

To calculate the difference between dates, the entered dates are first converted to Unix epoch time stamps, counting up from January 1, 1970. Then, the difference in seconds is converted back to days.

In order to return a positive value, the time stamps are then compared so the earlier time stamp is subtracted from the more recent one.

At the end, an object is generated which contains the number of days and a custom-generated text. It is stored into a variable so it can be alerted to the user.

The supported input date formats are "2024-12-02", "Dec 02, 2024" (with and without comma), and "02 Dec 2024". For example, calcDate("2021-01-01","2022-01-10") returns { total_days: 374, result: "1 year 9 days" }. The worded month can optionally be written out. Make sure not to miss quotation marks (both " and ' work), so the entered dates are interpreted as strings.

var calcDate_data = {}; // initializing data object

function calcDate(date1, date2){

// memorize input to detect invalid input
calcDate_data.input1 = date1;
calcDate_data.input2 = date2;

// initiate date object
var dt_date1 = new Date(date1);
var dt_date2 = new Date(date2);
// get the time stamp
date1 = dt_date1.getTime();
date2 = dt_date2.getTime();

var calc;
// check which time stamp is greater
if (date1 > date2){
    calc = new Date(date1 - date2) ;
} else {
    calc = new Date(date2 - date1) ;
}

// retrieve the date, month and year
var calc_format_tmp = calc.getDate() + '-' + (calc.getMonth()+1)+ '-'+calc.getFullYear();
// convert to an array and store
var calc_format = calc_format_tmp.split("-");
// subtract each member of our array from the default date
var days_passed = parseInt(Math.abs(calc_format[0]) - 1);
var months_passed = parseInt(Math.abs(calc_format[1]) - 1);
var years_passed = parseInt(Math.abs(calc_format[2] -   1970));

// set up custom text
var years_plural = ["year", "years"];
var months_plural = ["month", "months"];
var days_plural = ["day", "days"]; 

// convert to days and sum together
var total_days = (years_passed * 365) + (months_passed * 30.417) + days_passed;

// display result with custom text
var result = ""; // declare string
if (years_passed == 1) result+=years_passed+' ' + years_plural[0];
if (years_passed > 1 ) result+=years_passed+' ' + years_plural[1];
if (result != "" && months_passed > 0 ) result+=' '; // add space only if anything follows
if (months_passed == 1) result+=months_passed+' ' + months_plural[0];
if (months_passed > 1 ) result+=months_passed+' ' + months_plural[1];
if (result != "" && days_passed > 0 ) result+=' ';
if (days_passed == 1) result+=days_passed+' ' + days_plural[0];
if (days_passed > 1 ) result+=days_passed+' ' + days_plural[1];

// put last result into object so it can be alerted to the user
calcDate_data.last_result = {
    "total_days" : Math.round(total_days),
    "text" :  result
};

// message returned when an invalid date was input
if (! calcDate_data.last_result.text || calcDate_data.input1==null || calcDate_data.input2==null) {
	calcDate_data.last_result.text = "At least one input date is invalid.";
}

// return the result
return calcDate_data.last_result;

}

// example use
calcDate(
	// prompt user to enter two dates
	prompt("Date calculator \n Enter start date"),
	prompt("Date calculator \n Enter end date")
);
alert(calcDate_data.last_result.text); // alert result text