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Managing calendar dates and times

All of the calendar date and time management functions in ActionScript 3.0 are concentrated in the top-level Date class. The Date class contains methods and properties that let you handle dates and times in either Coordinated Universal Time (UTC) or in local time specific to a time zone. UTC is a standard time definition that is essentially the same as Greenwich Mean Time (GMT).


Creating Date objects

The Date class boasts one of the most versatile constructor methods of all the core classes. You can invoke it four different ways.

First, if given no parameters, the Date() constructor returns a Date object containing the current date and time, in local time based on your time zone. Here's an example:

var now:Date = new Date();

Second, if given a single numeric parameter, the Date() constructor treats that as the number of milliseconds since January 1, 1970, and returns a corresponding Date object. Note that the millisecond value you pass in is treated as milliseconds since January 1, 1970, in UTC. However, the Date object shows values in your local time zone, unless you use the UTC-specific methods to retrieve and display them. If you create a new Date object using a single milliseconds parameter, make sure you account for the time zone difference between your local time and UTC. The following statements create a Date object set to midnight on the day of January 1, 1970, in UTC:

var millisecondsPerDay:int = 1000 * 60 * 60 * 24;
// gets a Date one day after the start date of 1/1/1970
var startTime:Date = new Date(millisecondsPerDay);

Third, you can pass multiple numeric parameters to the Date() constructor. It treats those parameters as the year, month, day, hour, minute, second, and millisecond, respectively, and returns a corresponding Date object. Those input parameters are assumed to be in local time rather than UTC. The following statements get a Date object set to midnight at the start of January 1, 2000, in local time:

var millenium:Date = new Date(2000, 0, 1, 0, 0, 0, 0);

Fourth, you can pass a single string parameter to the Date() constructor. It will try to parse that string into date or time components and then return a corresponding Date object. If you use this approach, it's a good idea to enclose the Date() constructor in a try..catch block to trap any parsing errors. The Date() constructor accepts a number of different string formats, as listed in the ActionScript 3.0 Language and Components Reference. The following statement initializes a new Date object using a string value:

var nextDay:Date = new Date("Mon May 1 2006 11:30:00 AM");

If the Date() constructor cannot successfully parse the string parameter, it will not raise an exception. However, the resulting Date object will contain an invalid date value.

Getting time unit values

You can extract the values for various units of time within a Date object using properties or methods of the Date class. Each of the following properties gives you the value of a time unit in the Date object:

  • The fullYear property
  • The month property, which is in a numeric format with 0 for January up to 11 for December
  • The date property, which is the calendar number of the day of the month, in the range of 1 to 31
  • The day property, which is the day of the week in numeric format, with 0 standing for Sunday
  • The hours property, in the range of 0 to 23
  • The minutes property
  • The seconds property
  • The milliseconds property

In fact, the Date class gives you a number of ways to get each of these values. For example, you can get the month value of a Date object in four different ways:

  • The month property
  • The getMonth() method
  • The monthUTC property
  • The getMonthUTC() method

All four ways are essentially equivalent in terms of efficiency, so you can use whichever approach suits your application best.

The properties just listed all represent components of the total date value. For example, the milliseconds property will never be greater than 999, since when it reaches 1000 the seconds value increases by 1 and the milliseconds property resets to 0.

If you want to get the value of the Date object in terms of milliseconds since January 1, 1970 (UTC), you can use the getTime() method. Its counterpart, the setTime() method, lets you change the value of an existing Date object using milliseconds since January 1, 1970 (UTC).

Performing date and time arithmetic

You can perform addition and subtraction on dates and times with the Date class. Date values are kept internally in terms of milliseconds, so you should convert other values to milliseconds before adding them to or subtracting them from Date objects.

If your application will perform a lot of date and time arithmetic, you might find it useful to create constants that hold common time unit values in terms of milliseconds, like the following:

public static const millisecondsPerMinute:int = 1000 * 60;
public static const millisecondsPerHour:int = 1000 * 60 * 60;
public static const millisecondsPerDay:int = 1000 * 60 * 60 * 24;

Now it is easy to perform date arithmetic using standard time units. The following code sets a date value to one hour from the current time using the getTime() and setTime() methods:

var oneHourFromNow:Date = new Date();
oneHourFromNow.setTime(oneHourFromNow.getTime() + millisecondsPerHour);

Another way to set a date value is to create a new Date object using a single milliseconds parameter. For example, the following code adds 30 days to one date to calculate another:

// sets the invoice date to today's date
var invoiceDate:Date = new Date();

// adds 30 days to get the due date
var dueDate:Date = new Date(invoiceDate.getTime() + (30 * millisecondsPerDay));

Next, the millisecondsPerDay constant is multiplied by 30 to represent 30 days' time and the result is added to the invoiceDate value and used to set the dueDate value.

Converting between time zones

Date and time arithmetic comes in handy when you want to convert dates from one time zone to another. So does the getTimezoneOffset() method, which returns the value in minutes by which the Date object's time zone differs from UTC. It returns a value in minutes because not all time zones are set to even-hour increments--some have half-hour offsets from neighboring zones.

The following example uses the time zone offset to convert a date from local time to UTC. It does the conversion by first calculating the time zone value in milliseconds and then adjusting the Date value by that amount:

// creates a Date in local time
var nextDay:Date = new Date("Mon May 1 2006 11:30:00 AM");

// converts the Date to UTC by adding or subtracting the time zone offset
var offsetMilliseconds:Number = nextDay.getTimezoneOffset() * 60 * 1000;
nextDay.setTime(nextDay.getTime() + offsetMilliseconds);


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