Date from String in Swift but it becomes the previous day - swift

Question

I am trying to create a day from a string (mSince) :
let dateFormatter = DateFormatter()
dateFormatter.dateFormat = "yyyy-MM-dd"
let newDate = dateFormatter.date(from:mSince as! String)!
and the print values are:
mSince = Optional(2021-04-25)
newDate = 2021-04-24 21:00:00 +0000
I could not figure out why newDate becomes the previous day.
Thank you!

Answer 1

Below is an example of how to create dates from multiple UTC timezones and an example of how to get the delta (in seconds) between the dates:
extension Date {
static func - (lhs: Date, rhs: Date) -> TimeInterval {
return lhs.timeIntervalSinceReferenceDate - rhs.timeIntervalSinceReferenceDate
}
}
let utcDateFormatter = DateFormatter()
utcDateFormatter.dateStyle = .medium
utcDateFormatter.timeStyle = .medium
// Set the timeZone to the device’s local time zone.
utcDateFormatter.timeZone = TimeZone(abbreviation: "UTC")
let date1 = Date()
print(utcDateFormatter.string(from: date1))
// Parsing a string date
let dateString1 = "May 11, 2020 at 4:23:11 AM"
let utcDate1 = utcDateFormatter.date(from: dateString1)
// Set a date for a different timeZone (2 hours difference)
utcDateFormatter.timeZone = TimeZone(secondsFromGMT: 3600)
// Printing a Date
let date2 = Date()
print(utcDateFormatter.string(from: date2))
// Parsing a string representing a date
let dateString2 = "May 11, 2020 at 4:23:11 AM"
let utcDate2 = utcDateFormatter.date(from: dateString2)
let diff = utcDate1! - utcDate2!
print(diff)