return the right dateString and dateFormat of a ISO8601 to use it in DateFormatter, validated by regular expression, so, you don't need to fight again with those formats

DateFormatterISO8601Regex

func formatFrom(_ value:String) -> (dateString:String?, dateFormat:String?) {
        
        var regexString = "\\A(\\d{4})-(\\d{2})-(\\d{2})T(\\d{2}):(\\d{2})" // Mandatory - YYYY-MM-DDTHH:mm
        regexString = regexString + ":?(\\d{2})?"                             // Optional - :ss
        regexString = regexString + "[.]?(\\d{1,6})?"                         // Optional - .nnnnnn
        regexString = regexString + "([+-])?(\\d{2})?:?(\\d{2})?|Z"           // Optional -[+-]hh:mm or Z
        regexString = regexString + "\\z"
        
        let regex: NSRegularExpression?
        var matchesResult: [NSTextCheckingResult]?
        do{
            regex = try NSRegularExpression (pattern: regexString, options: NSRegularExpression.Options.caseInsensitive)
            
            matchesResult = regex!.matches(in:value, options: NSRegularExpression.MatchingOptions.reportCompletion, range:NSMakeRange(0, value.characters.count))
        } catch{
            return (nil, nil)
        }
        
        guard let matches = matchesResult, matches.count > 0 else {
            return (nil, nil)
        }
        
        var YYYY: String?, MM: String?, DD: String?, hh: String?, mm: String?, ss: String?, nn: String?, sign: String?, Zhh: String?, Zmm: String?
        var tempRange: NSRange!
        
        
        //this function is to convert a NSRange to Range<String.Index>
        let convertRange:(_ nsRange: NSRange, _ text:String) -> Range<String.Index>? = { nsRange, text in
            guard
                let from16 = text.utf16.index(text.utf16.startIndex, offsetBy: nsRange.location, limitedBy: text.utf16.endIndex),
                let to16 = text.utf16.index(from16, offsetBy: nsRange.length, limitedBy: text.utf16.endIndex),
                let from = String.Index(from16, within: text),
                let to = String.Index(to16, within: text)
                else { return nil }
            return from ..< to
        }
        
        
        for match in matches {
            let matchRange = match.range
            let matchCount = match.numberOfRanges - 1
            var idx = 0 //components start from 1, 0 is the complete string
            
            let checkAndAssign:()->(String?) = {
                if idx < matchCount {
                    idx = idx + 1
                    tempRange = match.rangeAt(idx)
                    var substring:String? = tempRange.location != NSNotFound ? value.substring(with: convertRange(tempRange, value)!) : nil
                    return substring
                }
                return nil
            }
            
            YYYY = checkAndAssign()
            MM = checkAndAssign()
            DD = checkAndAssign()
            hh = checkAndAssign()
            mm = checkAndAssign()
            ss = checkAndAssign()
            nn = checkAndAssign()
            sign = checkAndAssign()
            Zhh = checkAndAssign()
            Zmm = checkAndAssign()
        }
        
        var dateString:String = ""
        var formatString:String = ""
        
        if let YYYY = YYYY {
            dateString = dateString + YYYY + "-"
            formatString = formatString + "yyyy" + "-"
        }
        
        if let MM = MM {
            dateString = dateString + MM + "-"
            formatString = formatString + "MM" + "-"
        }
        
        if let DD = DD {
            dateString = dateString + DD
            formatString = formatString + "dd"
        }
        
        if let hh = hh {
            dateString = dateString + "T" + hh
            formatString = formatString + "'T'" + "HH"
        }
        
        if let mm = mm {
            dateString = dateString + ":" + mm
            formatString = formatString + ":" + "mm"
        }
        
        if let ss = ss {
            dateString = dateString + ":" + ss
            formatString = formatString + ":" + "ss"
        }
        if let nn = nn {
            dateString = dateString + "." + nn
            formatString = formatString + "." + String(repeating: "S", count: nn.characters.count)
        }
        if let sign = sign {
            dateString = dateString  + sign
            formatString = formatString + "Z"
        }
        if let Zhh = Zhh {
            dateString = dateString + Zhh
            formatString = formatString + "ZZ"
        }
        
        if let Zmm = Zmm {
            dateString = dateString + Zmm
            formatString = formatString + "ZZ"
        }
        
        return (dateString, formatString)
    }