Update timestamp creation to use UTCtime for RFC5280 compliance

% /Applications/YubiKey\ Manager.app/Contents/MacOS/ykman piv certificates export F9 -F PEM /tmp/f9.crt

% openssl verify -CAfile /tmp/f9.crt /tmp/f9.crt                                                                                  
/tmp/f9.crt: OK

% openssl x509 -in /tmp/f9.crt -noout -text
Certificate:
    Data:
        Version: 3 (0x2)
        Serial Number:
            74:d9:c7:51:fc:44:18:87:97:7b:34:59:87:f4:f2:40
    Signature Algorithm: ecdsa-with-SHA256
        Issuer: CN=PIVApplet Attestation
        Validity
            Not Before: Jan  1 00:00:00 2018 GMT
            Not After : Jan  1 00:00:00 2050 GMT
        Subject: CN=PIVApplet Attestation
        Subject Public Key Info:
            Public Key Algorithm: id-ecPublicKey
                Public-Key: (256 bit)
                pub: 
                    04:8a:31:c1:95:54:a6:66:a7:61:02:da:a1:f8:d3:
                    9d:f0:6c:5b:dc:bd:e0:8b:e0:7f:af:79:8a:dd:57:
                    fb:17:2e:5e:c2:94:07:00:2a:80:86:2e:8b:36:f7:
                    6b:56:67:7b:0d:ee:d4:df:dc:85:6a:8b:d9:ae:f4:
                    e9:ac:21:66:a1
                ASN1 OID: prime256v1
                NIST CURVE: P-256
        X509v3 extensions:
            1.3.6.1.4.1.41482.3.3: 
                ...
            1.3.6.1.4.1.41482.3.8: 
                ..
    Signature Algorithm: ecdsa-with-SHA256
         30:45:02:21:00:c1:a7:c4:5f:91:64:fe:50:4e:2b:e8:d3:9d:
         fe:e9:19:89:ad:0f:54:03:57:d4:4e:0c:d3:ab:03:38:79:61:
         3c:02:20:7b:70:78:5c:29:4d:20:3b:dd:0e:cc:95:38:7b:e8:
         42:0b:63:6f:b6:22:c2:0b:19:be:42:df:aa:2a:93:d9:ff
arekinath / PivApplet

Update timestamp creation to use UTCtime for RFC5280 compliance #56
