Why is the wrong time displayed after using date.toISOString()?

Question

JavaScript

I'm interested in this piece of code:

const handleClickDay = (value) =>{
    console.log(value)
    var valToStr = value.toISOString().slice(0,10)
    console.log(valToStr)

where value is the new Date() object. I receive date, I transform to isostring and I cut off seconds. but here are the miracles: in the console we get:

Wed Jun 02 2021 00:00:00 GMT + 0300 (Saudi Arabia, standard time)
2021-06-01

That is, the day from new Date is 1 more than after conversion. WHY?
Of course, I set up a crutch, add +1 day and everything works, but I would like to understand what's the matter.