How JavaScript calculates time

JavaScript is a very popular programming language that is widely used in web development, mobile application development, desktop application development and other fields. In JavaScript, time is a very important data type, and time can be represented and manipulated through JavaScript's built-in Date object. This article will discuss how JavaScript calculates time.

1. Time representation in JavaScript

In JavaScript, time is represented in the form of a Date object. The constructor of the Date object accepts an integer parameter or a specific set of year, month, day, hour, minute, second and millisecond values, representing the number of milliseconds that have passed since 00:00:00 UTC on January 1, 1970. For example, the following code creates a Date object representing 12:34:56 on September 1, 2021:

var date = new Date(2021, 8, 1, 12, 34, 56);

where the month starts counting from 0, so 8 represents September. You can also directly pass in the number of milliseconds:

var date = new Date(1630472096000);

The date objects created by the above two code snippets are equivalent.

2. Time operations in JavaScript

In JavaScript, you can perform basic addition and subtraction operations on Date objects to obtain new date and time objects. The following are common time operations:

1. Addition operation

The getTime() method of the Date object can return the date since January 1, 1970 00:00:00 UTC The number of milliseconds that have elapsed since the specified time. Therefore, by adding two Date objects, you will get the time difference between them (in milliseconds). Then add this time difference to another Date object to get a new Date object, which means adding the specified time difference. later time.

For example, the following code will create a Date object representing the next second at 12:34:56 on September 1, 2021:

var date = new Date(2021, 8, 1, 12, 34, 56);
var nextSecond = new Date(date.getTime() + 1000);

Among them, nextSecond is a date representing September 2021 Date object at 12:34:57 on the 1st.

Similarly, you can also use the setTime() method of the Date object to set the number of milliseconds in the specified time, for example:

var date = new Date(2021, 8, 1, 12, 34, 56);
date.setTime(date.getTime() + 1000);

The above code will set the date object to represent September 1, 2021 The time is 12:34:57.

2. Subtraction operation

Similarly, two Date objects can be subtracted to get the time difference between them. The following are common subtraction operations:

(1) Calculate the time difference (in milliseconds) between two Date objects:

var date1 = new Date(2021, 8, 1, 12, 34, 56);
var date2 = new Date(2021, 8, 2, 12, 34, 56);
var diff = date2.getTime() - date1.getTime();

The above code will get the value of diff to be 86400000, which is 24 Hours (246060*1000 milliseconds).

(2) Calculate the time difference (in milliseconds) between the current time and the specified time:

var date = new Date(2021, 8, 1, 12, 34, 56);
var now = new Date();
var diff = now.getTime() - date.getTime();

The above code will get the diff value as the current time and September 1, 2021 12 The time difference (in milliseconds) between :34:56.

3. Time interval calculation

JavaScript has some built-in functions for calculating time intervals, including getTime(), getFullYear(), getMonth(), getDate(), getDay(), getHours(), getMinutes(), getSeconds(), getMilliseconds(), etc.

For example, the following code calculates the time interval from 12:34:56 on September 1, 2021 to the current time:

var date = new Date(2021, 8, 1, 12, 34, 56);
var now = new Date();
var yearDiff = now.getFullYear() - date.getFullYear();
var monthDiff = now.getMonth() - date.getMonth();
var dayDiff = now.getDate() - date.getDate();
var hourDiff = now.getHours() - date.getHours();
var minuteDiff = now.getMinutes() - date.getMinutes();
var secondDiff = now.getSeconds() - date.getSeconds();
var millisecondDiff = now.getMilliseconds() - date.getMilliseconds();

The above code will get the current time and September 2021 The time interval between 12:34:56 on the 1st, in years, months, days, hours, minutes, seconds and milliseconds. These values ​​can be used in combination for time calculations based on specific needs.

3. Date formatting in JavaScript

In actual development, we usually need to convert the Date object into a specified date format to facilitate display or interaction with other systems. JavaScript provides some commonly used date formatting functions, as follows:

1. toDateString(): Converts a Date object to a date string in the format of Weekday Month Date Year, for example: Thu Apr 8 2021 .
2. toTimeString(): Convert the Date object to a time string in the format hh:mm:ss GMT 0800 (TimeZone), for example: 09:28:14 GMT 0800 (China Standard Time).
3. toLocaleDateString(): Convert the Date object to a localized date string, for example: April 8, 2021.
4. toLocaleTimeString(): Convert the Date object to a localized time string, for example: 9:28:14 AM.
5. toLocaleString(): Convert the Date object to a localized date time string, for example: April 8, 2021 9:28:14 AM.
6. toUTCString(): Convert the Date object to a UTC standard time string, for example: Thu, 08 Apr 2021 01:28:14 GMT.

In addition to the above functions, you can also use third-party libraries or manually write code to format date and time.

4. Conclusion

This article introduces how to calculate time in JavaScript, including time representation, addition and subtraction operations, time interval calculation and date formatting, etc. The time calculation function in JavaScript is very powerful and can be applied to various fields, such as web page dynamic effects, countdown functions, timestamp conversion, etc. By studying the content introduced in this article, readers can have a deeper understanding of JavaScript's time calculation related knowledge, so that it can be used more flexibly in practical applications.

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