JS setHours on date is not working correctly

Question

I have the following code to increment the hours in a date:

let timerExpireDate = new Date(countdownStartDate);
console.log(`original date is ${timerExpireDate}`);
console.log(`add on ${countdownHours} hours`);
timerExpireDate.setHours(timerExpireDate.getHours() + countdownHours);
console.log(`New date is ${timerExpireDate}`);

However it also seems to be incrementing the days by 6, here is the console log:

original date is Sun Jul 19 2020 16:36:39 GMT+0800 (Taipei Standard Time)
add on 2 hours
New date is Sat Jul 25 2020 18:36:39 GMT+0800 (Taipei Standard Time)

What am I doing wrong here?

Try running your code again in about... 10 minutes or so. See what happens. — Niet the Dark Absol, Jul 19 '20 at 08:49
You should provide a runnable snippet, with all values, next time — Griffin, Jul 19 '20 at 08:52
What are the values and type of `countdownStartDate`, `countdownHours` and how do they get their values? — iAmOren, Jul 19 '20 at 08:52
Does this answer your question? [Adding two numbers concatenates them instead of calculating the sum](https://stackoverflow.com/questions/14496531/adding-two-numbers-concatenates-them-instead-of-calculating-the-sum) — user120242, Jul 19 '20 at 08:54

Code · Accepted Answer · 2020-07-19T09:08:01.933

2

It is likely that countdownHours is of type string instead of number, so timerExpireDate.getHours() + countdownHours is '162' (6 days later) instead of 18.

The fix is to cast countdownHours to number first, like countdownHours = +countdownHours.

edited Jul 19 '20 at 09:08

answered Jul 19 '20 at 08:46

Code

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2

It is by *complete coincidence* that `162` is 6 days and 18 hours, making the hours *look* right but the day "mysteriously" changing. – Niet the Dark Absol Jul 19 '20 at 08:47
1

Might benefit from adding a runnable snippet showing why or a code block showing how to fix it – user120242 Jul 19 '20 at 09:02

JS setHours on date is not working correctly

1 Answers1