Javascript Date.UTC() function is off by a month? - javascript

I was playing around with Javascript creating a simple countdown clock when I came across this strange behavior:
var a = new Date(),
now = a.getTime(),
then = Date.UTC(2009,10,31),
diff = then - now,
daysleft = parseInt(diff/(24*60*60*1000));
console.log(daysleft );
The days left is off by 30 days.
What is wrong with this code?
Edit: I changed the variable names to make it more clear.

The month is zero-based for JavaScript.
Days and years are one-based.
Go figure.
UPDATE
The reason this is so, from the creator of JavaScript, is
JS had to "look like Java" only less so, be Java's dumb kid brother or boy-hostage sidekick. Plus, I had to be done in ten days or something worse than JS would have happened.
http://www.jwz.org/blog/2010/10/every-day-i-learn-something-new-and-stupid/#comment-1021

As Eric said, this is due to months being listed as 0-11 range.
This is a common behavior - same is true of Perl results from localtime(), and probably many other languages.
This is likely originally inherited from Unix's localtime() call.
(do "man localtime")
The reason is that days/years are their own integers, while months (as a #) are indexes of an array, which in most languages - especially C where the underlying call is implemented on Unix - starts with 0.

It's an old question but this is still a problem today (or a feature as some might say - and they are wrong).
JS is zero-based month, why? Because.
That means the months range from 0-11 (only the months, the others are normal)
How can you fix this? Add a month, obviously, BUUUUT:
Don't do this :
let date: Date = new Date();
date.setMonth(date.getMonth() + 1);
Why you might ask? Because it won't work as expected, Date in JS is terrible.
You have to make a ... let's call it not so beautiful function to translate the JS date to a normal date
formatJsDateToNormalDate(Date date): string | null {
if(date !== null) {
const realMonth: number = date.getMonth() + 1;
let month: string = (realMonth < 10) ? '0' + realMonth : String(realMonth);
let day: string = (date.getDate() < 10) ? '0' + date.getDate() : String(date.getDate());
return [date.getFullYear(), month, day].join('-');
} else {
return null;
}
Again, if you ask me this is the equivalent of hammering a screw, it's not the right way, but there is no right way here, it's a bug that has been going on for 27 years and more to come.

date1 = new Date();
//year, month, day [, hrs] [, min] [, sec]
date1 = new Date.UTC(date1.getFullYear(),date1.getMonth()+1,date1.getDate(),date1.getHours(),date1.getMinutes(),date1.getSeconds());
date2 = new Date();
date2 = date2.getTime();
alert(date1)
alert(date2)

Related

Age verification based on birthdate and current date

So, I need to calculate age by subtracting "todays" date from the converted date of an input field, entered by the user. Although it needs cleaned up, the below code works, I had to get creative as RN uses a different JS execution environment... see here.
My question, without adding the "+1" to this snippet "b.getMonth() + 1", the math on the date subtraction comes back 1 month off every time. When I add the "+1" it works like a charm, why? If it's a logical fix, I don't mind keeping the "+1," but I would surely like to know why the "+1" is necessary.
Also, totally open to improved solutions to this problem, keep in mind I had a much simpler function that worked great while debugger was open, once closed, it did not work, see the link above.
getVerifyBirthday(birthday) {
const b = new Date();
var verify = birthday.length;
const utc2 = Date.UTC(b.getFullYear(), b.getMonth() + 1, b.getDate());
if (verify === 10) {
const splitBirth = birthday.split('-');
var mydate = new Date(splitBirth[2], splitBirth[0], splitBirth[1]);
const a = mydate;
const utc1 = Date.UTC(a.getFullYear(), a.getMonth(), a.getDate());
const diffTime = Math.abs(utc2 - utc1);
const diffYears = (diffTime / (3.15576e+10));
this.setState({ diffYears: diffYears});
return diffYears >= 13;
} else {}
}
Update
I ended up refactoring my original function, leaving the (+1) on months due to months starting at 0, as mentioned below. I still had to split both dates, when I didn't split both, my age came back NaN/Undefined; not sure if this goes back to the RN execution environment vs browser, but I digress.
I'd make a few of my own suggestions over here:
there's no need to hussle with UTC dates (to make sure whether the user in his timezone has already reached the age of 13, assuming along the way that he or she was born in that same timezone ;)
there's no need to split mm-dd-yyyy date string to convert into Date, it may be parsed by new Date()
counting years as 365.25 days has certain error margin depending on the exact leap years quantity that passed since the user's birth year, instead whole years may be compared together with dates
To me, it makes more sense to decompose date strings into days, months and years and make decision based on full years difference minus 1 year (if the person didn't yet celebrate his/her birthday this year):
const today = new Date().toISOString().slice(0,10), // yyyy-mm-dd
birthday = '1982-06-21',
[bYear, bMonth, bDay] = birthday.split('-'),
[tYear, tMonth, tDay] = today.split('-'),
diffYears = tYear - bYear - (bMonth > tMonth || bDay > tDay ? 1 : 0)
console.log(diffYears)
.as-console-wrapper{min-height:100%;}
Months are zero-based in JavaScript Date objects. However, if you get a formatted string, they start from 1:
const date = new Date(2020, 1, 17); // 17th of February 2020
console.log("getMonth:", date.getMonth()); //month is 1
console.log("formatted:", date.toISOString()); //month is 2
So, actually what happens is that you're shifting both dates a month forward. This sort of works:
const originDate = new Date(2020, 1, 17); // 17th of February 2020
const originString = "2020-02-17".split("-");
const dateFromDate = new Date(originDate.getFullYear(), originDate.getMonth() + 1, originDate.getDate())
const dateFromString = new Date(originString[0], originString[1], originString[2])
console.log("dateFromDate:", dateFromDate); //month is 3
console.log("dateFromString:", dateFromString); //month is 3
When you do the subtraction it evens out but you can still run into an overflow of the date for months with different number of days:
const originDate = new Date(2020, 0, 31); // 31st of January 2020
const dateFromDate = new Date(originDate.getFullYear(), originDate.getMonth() + 1, originDate.getDate())
console.log("dateFromDate:", dateFromDate); // 1st of March 2020
This still works logically for most cases, however you are bound to run into a problem at some point if you shift months forward. So, instead you should be doing the opposite and subtracting 1 when converting a 1-based number into a Date object:
const originString = "2020-02-17".split("-");
const dateFromString = new Date(originString[0], originString[1] - 1, originString[2])
console.log("dateFromString:", dateFromString); //month is 2

Momentjs - Get most recent Friday

I'm trying to get the start of (12:00am, or, 00:00am) of the most recent Friday. This has been working:
moment().isoWeekday(5).startOf('day').toDate()
But it only works Friday->Sunday, on Monday morning it will then refer to the upcoming Friday, in which case this would work:
moment().add('-1', 'week').day(5).startOf('day').toDate()
but I need it be dynamic and done in one line if possible, to where I don't to perform any checks on the current day.
Is there a way to always get the most recent Friday? Regardless of what the current day is.
Edit I'm also trying to get this to return the current day (friday) if executed on a Friday.
If you don't want to use a library, it's pretty straight forward
var date = new Date();
while ( date.getDay() !== 5 ) date.setDate(date.getDate() -1);
console.log(date)
With moment
var date = moment();
var friday = date.day(date.day() >= 5 ? 5 :-2);
and if millisecond accuracy doesn't matter, you could call moment() twice to make it one line (but I would much raher use a variable)
var friday = moment().day(moment().day() >= 5 ? 5 :-2);
Check this:
var date = moment().utc().isoWeekday(5);
if(moment().day() < 5) {
date = date.add(-1, 'week');
}
console.log('Recent friday starts:', date.startOf('day').toDate());
console.log('Recent friday ends:', date.endOf('day').toDate());
<script src="https://cdnjs.cloudflare.com/ajax/libs/moment.js/2.15.1/moment.js"></script>

Convert milliseconds to years

I have a validator that checks if an user is at least 18 years old.
This is the check:
var res = /^([1-2]\d{3})\-(0[1-9]|1[012])\-(0[1-9]|[12][0-9]|3[01])\-([0-9]{4})$/.exec(str);
var todays_date = new Date();
var birth_date = null;
if (res != null) {
birth_date = new Date(res[1], res[2], res[3]);
if (todays_date - birth_date > 565633905872) {
565633905872 is 18 years in milliseconds but how do I convert it to years before so I can just do:
if (todays_date - birth_date => 18) {
The number you have quoted is not the number of milliseconds in 18 years. It's too small even if you pretend there are no leap years.
The simplest way to test if somebody is at least 18 years old is to initialise a date object to their birthday, then use .getFullYear() and .setFullYear() to directly set the year 18 years forward. Then compare that with the current date.
Note also that in JS dates the month is zero-based, so you probably want to use res[2] - 1 when creating the date object.
birth_date = new Date(res[1], res[2] - 1, res[3]);
birth_date.setFullYear(birth_date.getFullYear() + 18);
if (birth_date <= new Date()) {
Or given you are constructing the birth_date from individual year, month and day you could just do:
birthPlus18 = new Date(+res[1] + 18, res[2] - 1, res[3]);
if (birthPlus18 <= new Date()) {
(The leading + in +res[1] + 18 is not a typo, it converts the string extracted by your regex into a number so that you can add 18 to it. You don't need to do the same thing for res[2] - 1 because - automatically converts both operands.)
Note also that your regex will happily allow dates that specify a day that is too high for the month, e.g., Feb 30 or Jun 31.
There are better ways of checking this (see the answer of "nnnnnn"). But your question wasn't about a better way but, how you could convert to years.
You could write a function that does that, example:
function convertmili( mSeconds )
{
return mSeconds / 31536000000;
}
The output of this function is still far from ideal, because your example would output: 17.9361334941654
So we could clean it up a bit:
function convertmili( mSeconds )
{
var checkYear = Math.floor(mSeconds / 31536000000);
return checkYear;
}
With this function, your example would output 17 and then you can check it the way you wanted.
Divide your millisecond value by 31536000000 you get number of years
http://www.convertunits.com/from/milliseconds/to/year

JavaScript time question

I am new to JavaScript but need to run a check to make sure it is daylight. I am using yahoo's weather API to pull sunrise and sunset. I'm just a little confused as to the best approach for comparing its results to the current time.
I am confused because it returns a time like sunset: '9:01 pm'. bsince there is a PM it is text. I can't think of a good way to compare it to the current time... RegExp, then convert to an integer maybe?
What would be the best approach to this, and why (sorry I'm trying to learn)?
Thanks in advance for any help.
Create a new Date() with the info from yahoo's api, then compare Date.now() with sunsetDate.getTime() and sunriseDate.getTime().
Passing today's date in mm/dd/yyyy format with the time as '9:01 pm' to the Date constructor will give you a valid date.
var today = new Date();
today = [today.getMonth()+1, today.getDate(), today.getFullYear()].join('/');
var yahooSunrise = '5:45 am';
var yahooSunset = '9:01 pm';
var sunrise = new Date(today + ' ' + yahooSunrise).getTime();
var sunset = new Date(today + ' ' + yahooSunset).getTime();
var now = Date.now();
var isDaylight = (now > sunrise && now < sunset);
This would work with something like this, but maybe you might need to change the timings to suite a particular climate:
function getNow() {
var now = new Date
if (now.getHours() < 5) { return "Could be still dark";}
else if (now.getHours() < 9) {return "Definitely day time";}
else if (now.getHours() < 17) { return "Definitely day time"; }
else {return "It gets dark now";}
}
alert(getNow());
One quick approach is to turn both the current time of day and the time you get back from yahoo into the value "minutes since the beginning of the day."
For example, if Yahoo gives you 9:01pm, use the pm to turn the time into 21:01. That is
21*60 + 1 = 1260 + 1 = 1261 minutes since the beginning of the day
Do this for both sunrise and suset. Then get the current time with
new Date()
and do the same kind of thing.
Then just do integer comparisons!
Hope that helps.
This sounds like a good candiidate for a regular expression on the data you get back from the service.
Something like (\d{1,2}):(\d{2})\s(AM|PM) will give you 3 capture groups.
1: The hour (1 or 2 digits)
2: The Minute (2 digits)
3: Either string "AM" or "PM"
You can then use these to parse out the actual time as integer hour and minute to compare to the current time.

the closest Sunday before given date with JavaScript

I need to know the date for last Sunday for given date in php & javascript
Let's have a function give_me_last_Sunday
give_me_last_Sunday('20110517') is 20110515
give_me_last_Sunday('20110604') is 20110529
The full backup is done on Sundays = weekly. If I want to restore daily backup I need full (weekly) and daily backup. I need to copy backup files before restoring to temp directory so I restoring daily backup I need to know what weekly backup file I need to copy along the daily file.
My thought was to get Julian representation (or something similar) for the given date and then subtract 1 and check if it is Sunday ... Not sure if this is the best idea and how to convert given date into something I can subtract.
Based on Thomas' effort, and provided the input string is exactly the format you specified, then:
function lastSunday(d) {
var d = d.replace(/(^\d{4})(\d{2})(\d{2}$)/,'$1/$2/$3');
d = new Date(d);
d.setDate(d.getDate() - d.getDay());
return d;
}
Edit
If I were to write that now, I'd not depend on the Date object parsing the string but do it myself:
function lastSunday(s) {
var d = new Date(s.substring(0,4), s.substring(4,6) - 1, s.substring(6));
d.setDate(d.getDate() - d.getDay());
return d;
}
While the format yyyy/mm/dd is parsed correctly by all browsers I've tested, I think it's more robust to stick to basic methods. Particularly when they are likely more efficient.
Ok so this is for JavaScript only. You have an input that you need to extract the month, date, and year from. The following is just partly an answer then on how to get the date:
<script type="text/javascript">
var myDate=new Date();
myDate.setFullYear(2011,4,16)
var a = myDate.getDate();
var t = myDate.getDay();
var r = a - t;
document.write("The date last Sunday was " + r);
</script>
So the setFullYear function sets the myDate to the date specified where the first four digits is the year, the next are is the month (0= Jan, 1= Feb.,...). The last one is the actually date. Then the above code gives you the date of the Sunday before that. I am guessing that you can add more code to get the month (use getMonth() method). Here are a few links that might be helpful
http://www.w3schools.com/js/js_obj_date.asp
http://www.w3schools.com/jsref/jsref_setFullYear.asp
http://www.w3schools.com/jsref/jsref_getMonth.asp
(You can probably find the other functions that you need)
I hope this helps a bit even though it is not a complete answer.
Yup and strtotime has been ported to JS for eg http://phpjs.org/functions/strtotime:554 here.
final code (big thanks to #Thomas & #Rob)
function lastSunday(d) {
var d = d.replace(/(^\d{4})(\d{2})(\d{2}$)/,'$1/$2/$3');
d = new Date(d);
d.setDate(d.getDate() - d.getDay());
year = d.getFullYear()+'';
month = d.getMonth()+1+'';
day = d.getDate()+'';
if ( month.length == 1 ) month = "0" + month; // Add leading zeros to month and date if required
if ( day.length == 1 ) day = "0" + day;
return year+month+day;
}

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