Question

JavaScript has a standard object type for representing dates—or rather, points in time. It is called Date . If you simply create a date object using new , you get the current date and time.

console.log(new Date());
// → Wed Dec 04 2013 14:24:57 GMT+0100 (CET)

You can also create an object for a specific time.

console.log(new Date(2009, 11, 9));
// → Wed Dec 09 2009 00:00:00 GMT+0100 (CET)
console.log(new Date(2009, 11, 9, 12, 59, 59, 999));
// → Wed Dec 09 2009 12:59:59 GMT+0100 (CET)

JavaScript uses a convention where month numbers start at zero (so December is 11), yet day numbers start at one. This is confusing and silly. Be careful.

The last four arguments (hours, minutes, seconds, and milliseconds) are optional and taken to be zero when not given.

Timestamps are stored as the number of milliseconds since the start of 1970, using negative numbers for times before 1970 (following a convention set by “Unix time”, which was invented around that time). The getTime method on a date object returns this number. It is big, as you can imagine.

console.log(new Date(2013, 11, 19).getTime());
// → 1387407600000
console.log(new Date(1387407600000));
// → Thu Dec 19 2013 00:00:00 GMT+0100 (CET)

If you give the Date constructor a single argument, that argument is treated as such a millisecond count. You can get the current millisecond count by creating a new Date object and calling getTime on it but also by calling the Date.now function.

Date objects provide methods like getFullYear , getMonth , getDate , getHours , getMinutes , and getSeconds to extract their components. There’s also getYear , which gives you a rather useless two-digit year value (such as 93 or 14 ).

Putting parentheses around the parts of the expression that we are interested in, we can now easily create a date object from a string.

function findDate(string) {
  var dateTime = /(\d{1,2})-(\d{1,2})-(\d{4})/;
  var match = dateTime.exec(string);
  return new Date(Number(match[3]),
                  Number(match[2]) - 1,
                  Number(match[1]));
}
console.log(findDate("30-1-2003"));
// → Thu Jan 30 2003 00:00:00 GMT+0100 (CET)

This is a book about getting computers to do what you want them to do. Computers are about as common as screwdrivers today, but they contain a lot more hidden complexity and thus are harder to operate and understand. To many, they remain alien, slightly threatening things.