我如何将这些伪代码转换为可工作的js [不用担心结束日期是从哪里来的，除了它是有效的JavaScript日期之外]。

var myEndDateTime = somedate;  //somedate is a valid js date  
var durationInMinutes = 100; //this can be any number of minutes from 1-7200 (5 days)

//this is the calculation I don't know how to do
var myStartDate = somedate - durationInMuntes;

alert("The event will start on " + myStartDate.toDateString() + " at " + myStartDate.toTimeString());

一旦知道这一点：

• 您可以Date通过调用自1970年1月1日以来的毫秒数的构造函数来创建一个。
• 在valueOf()一个Date是自1970年1月1日的毫秒数
• 60,000一分钟有毫秒：-]

...并不难。

在下面的代码中，Date通过从中减去适当的毫秒数来创建一个新的myEndDateTime：

var MS_PER_MINUTE = 60000;
var myStartDate = new Date(myEndDateTime - durationInMinutes * MS_PER_MINUTE);

您还可以使用get和set分钟来实现它：

var endDate = somedate;

var startdate = new Date(endDate);

var durationInMinutes = 20;

startdate.setMinutes(endDate.getMinutes() - durationInMinutes);

一切都只是勾号，不需要记住方法 ...

var aMinuteAgo = new Date( Date.now() - 1000 * 60 );

要么

var aMinuteLess = new Date( someDate.getTime() - 1000 * 60 );

更新

在使用momentjs之后，我不得不说这是一个很棒的库，您应该检查一下。的确，滴答声在许多情况下都可以使您的代码变得非常小，并且您应该根据需要执行的操作使代码尽可能小。但是对于任何复杂的事情，请使用momentjs。

moment.js有一些非常好的便捷方法来操作日期对象

该.subtract方法，允许用户从一个日期减去一定量的时间单位，通过提供量和TIMEUNIT字符串。

var now = new Date();
// Sun Jan 22 2017 17:12:18 GMT+0200 ...
var olderDate = moment(now).subtract(3, 'minutes').toDate();
// Sun Jan 22 2017 17:09:18 GMT+0200 ...

这是我发现的：

//First, start with a particular time
var date = new Date();

//Add two hours
var dd = date.setHours(date.getHours() + 2);

//Go back 3 days
var dd = date.setDate(date.getDate() - 3);

//One minute ago...
var dd = date.setMinutes(date.getMinutes() - 1);

//Display the date:
var monthNames = ["January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December"];
var date = new Date(dd);
var day = date.getDate();
var monthIndex = date.getMonth();
var year = date.getFullYear();
var displayDate = monthNames[monthIndex] + ' ' + day + ', ' + year;
alert('Date is now: ' + displayDate);

资料来源：

http://www.javascriptcookbook.com/article/Perform-date-manipulations-based-on-adding-or-subtracting-time/

https://stackoverflow.com/a/12798270/1873386

var date=new Date();

//here I am using "-30" to subtract 30 minutes from the current time.
var minute=date.setMinutes(date.getMinutes()-30); 

console.log(minute) //it will print the time and date according to the above condition in Unix-timestamp format.

您可以使用。new Date()例如将Unix时间戳转换为常规时间。

var extract=new Date(minute)
console.log(minute)//this will print the time in the readable format.

尝试如下：

var dt = new Date();
dt.setMinutes( dt.getMinutes() - 20 );
console.log('#####',dt);

这就是我所做的：在Codepen上查看

var somedate = 1473888180593;
var myStartDate;
//var myStartDate = somedate - durationInMuntes;

myStartDate = new Date(dateAfterSubtracted('minutes', 100));

alert("The event will start on " + myStartDate.toDateString() + " at " + myStartDate.toTimeString());

function dateAfterSubtracted(range, amount){
    var now = new Date();
    if(range === 'years'){
        return now.setDate(now.getYear() - amount);
    }
    if(range === 'months'){
        return now.setDate(now.getMonth() - amount);
    }
    if(range === 'days'){
        return now.setDate(now.getDate() - amount);
    }
    if(range === 'hours'){
        return now.setDate(now.getHours() - amount);
    }
    if(range === 'minutes'){
        return now.setDate(now.getMinutes() - amount);
    }
    else {
        return null;
    }
}

使用此功能扩展Date类

// Add (or substract if value is negative) the value, expresed in timeUnit
// to the date and return the new date.
Date.dateAdd = function(currentDate, value, timeUnit) {

    timeUnit = timeUnit.toLowerCase();
    var multiplyBy = { w:604800000,
                     d:86400000,
                     h:3600000,
                     m:60000,
                     s:1000 };
    var updatedDate = new Date(currentDate.getTime() + multiplyBy[timeUnit] * value);

    return updatedDate;
};

因此，您可以在任意日期中添加或减去许多分钟，秒，小时，天...。

add_10_minutes_to_current_date = Date.dateAdd( Date(), 10, "m");
subs_1_hour_to_a_date = Date.dateAdd( date_value, -1, "h");