dateFormater returns nil on Specific date

Do you really run this in playground ?

Your date string is wrong: do not include ' before and after T

This works:

let birth = "1991-03-24T00:00:00"
func convertStringToDate(dateString : String) -> Date {
        let dateFormatter : DateFormatter = DateFormatter()
        dateFormatter.locale = Locale(identifier: "en_US_POSIX")
        dateFormatter.dateFormat = "yyyy-MM-dd'T'HH:mm:ss"
        dateFormatter.timeZone = TimeZone(identifier: "UTC") // Need to define TimeZone
        if let date = dateFormatter.date(from: dateString) {  // No parenthesis needed here around dateString
            return date
        }
        return Date()
    }
let birthDate = convertStringToDate(dateString: birth)
print("birthDate", birthDate)

.

And gives:

birthDate 1991-03-24 00:00:00 +0000

Notes:

• labels as DateString should start with lowercase
• You need to define TimeZone, otherwise you may get a different result depending on your locale

If you get the String with single quoted T,

let birth = "1991-03-24'T'00:00:00"

you have 2 options:

• remove the single quotes

var birth = "1991-03-24'T'00:00:00"
birth = birth.replacingOccurrences(of: "'", with: "")

• change the dateFormat, by adding single quote (ned to escape it, hence '' before and after 'T' as:

        dateFormatter.dateFormat = "yyyy-MM-dd'''T'''HH:mm:ss"

I could have found a few timezones where date(from:) returns nil for "1991-03-24T00:00:00". For example, Asia/Jerusalem.

I'm not sure what would be the reason for this specific case, but historically, some ranges of local date & time may be missing from the calendar system of some regions.

You may need to find a better way to represent a date using Date for such regions.