I know Paul say Dates are hard (not even the half of it) but wondering if anyone could tell why this happens

I have extension on Date to help with dates

extension Date {
    var startDateOfMonth: Date {
        guard let date = Calendar.current.date(from: Calendar.current.dateComponents([.year, .month], from: self)) else {
            fatalError("Unable to get start date from date")
        }
        return date
    }

    var endDateOfMonth: Date {
        guard let date = Calendar.current.date(byAdding: DateComponents(month: 1, day: -1), to: self.startDateOfMonth) else {
            fatalError("Unable to get end date from date")
        }
        return date
    }

    var dayNumber: Int {
        let components = Calendar.current.dateComponents([.day], from: self)
        guard let day = components.day else {
            fatalError("Unable to get day from date")
        }
        return day
    }
}

Then you use

let startDate = Date.now.startDateOfMonth
print(startDate) // prints 2022-03-01 00:00:00 +0000

let endDate = Date.now.endDateOfMonth
print(endDate) // prints 2022-03-30 23:00:00 +0000

let endDay = endDate.dayNumber
let startDay = startDate.dayNumber
print(startDay, endDay) // print 1 31

Why does endDate print 30th of month however the endDay is 31?

Could be an edge case of Daylight saving time this month. We're going to add one hour and then it would match.