Javascript Date - getDate gives different results in different years

Question

I have two date strings - same month, day and time, but different year (2017, 1970). When I convert them to Date object and then use getDate() function, I get different day.

var d1 = '1970-05-11T22:00:00.000Z';
var d2 = '2017-05-11T22:00:00.000Z';
console.log(new Date(d1), new Date(d1).getDate()); // day 11
console.log(new Date(d2), new Date(d2).getDate()); // day 12

Why is that so and how can I control this behavior?

Answer 1

Your profile indicates that you're in the Czech Republic. getDate returns values for the local time. In 2017, according to timeanddate.com, Daylight Savings was observed and was in effect in May. However, it also says that during 1970:

Daylight Saving Time (DST) Not Observed in Year 1970

Prague observed Central European Time (CET) all year.

DST was not in use in 1970.

The 22:00 in your UTC times is close enough to midnight for getDate to return the next day in your timezone, since it was the 12th local time in 2017, but still the 11th in 1970.

You can fix this by using getUTCDate instead, which returns 11 for both:

var d1 = '1970-05-11T22:00:00.000Z';
var d2 = '2017-05-11T22:00:00.000Z';
console.log(new Date(d1), new Date(d1).getUTCDate()); // day 11
console.log(new Date(d2), new Date(d2).getUTCDate()); // day 11