date-fns日期在format（）之后不同

Question

If I create a date using date-fns addYears() I see the date as expected:

let d = addYears(new Date('2015-01-01), 1) // 2016-01-01T00:00:00.000Z

However, when I use format() to output as a string, I lose a day:

let f = format(d, 'YYYY-MM-DD')

// expected output: 2016-01-01
// actual output: 2015-12-31

Is this a bug in format() or the expected output? If so, is it possible to workaround this?

Answer 1

I was just discussing this with someone. If you append T00:00 (explicitly setting the hours and minutes) to the date string it stores the correct date. If the hours and minutes are omitted the time zone offset, date-times are interpreted as user local time. When you omit the time altogether, dates are interpreted as UTC.

So foo = new Date('2015-01-01T00:00') should store the correct date, then use the format.() function as you are, it will output the correct date.

let d = addYears(new Date("2015-01-01T00:00"), 1);
let f = format(d, "YYYY-MM-DD");

will output ---> 2016-01-01