Is it possible to checkout the expiration time of a SSL key?

Keys do not expire. Certificates do. Certificates are binary data. They are not human-readable without tools.

Depending on how the certificate as encoded, you can just rename it to .cer and open it on Windows. It’ll then show you all data that’s in the certificate.

With OpenSSL, you can inspect certificates on the command line:

openssl x509 -text -noout -in cert.pem

OpenSSL is available for basically everything, but you may have to install it first.

You’ll get something like this:

Certificate:
    Data:
        Version: 3 (0x2)
        Serial Number:
            04:11:24:0f:88:1e:34:22:06:94:05:d0:74:e5:be:96:00:ec
    Signature Algorithm: sha256WithRSAEncryption
        Issuer: C=US, O=Let's Encrypt, CN=R3
        Validity
            Not Before: May 16 13:25:30 2021 GMT
            Not After : Aug 14 13:25:30 2021 GMT
        Subject: CN=*.stackexchange.com
        Subject Public Key Info:
            Public Key Algorithm: rsaEncryption
                Public-Key: (2048 bit)
                Modulus:
                    00:b6:91:0f:2e:ad:e9:c1:d3:44:32:5a:a3:2d:f7:
                    ca:2f:25:6e:6b:e0:ca:39:d9:63:36:1e:b7:d1:21:
                    ae:cb:43:12:aa:11:80:dc:3b:30:a9:7b:02:ed:66:
                    c5:42:ea:b3:9b:61:5d:17:2f:4f:36:d4:fd:7e:df:
                    ae:da:97:7d:23:e0:e0:f4:be:19:83:9e:a3:1b:2b:
                    32:a3:11:74:40:6c:dd:e3:ef:20:ee:2d:dc:0f:c2:
                    49:ce:89:fe:b2:fb:5f:c4:66:55:b5:4e:8b:23:2a:
                    79:33:2b:e7:94:7f:5d:2a:d8:ea:45:11:35:63:e5:
                    b6:69:b6:6d:b4:05:50:f8:15:76:36:6c:97:c6:d8:
                    61:6d:91:18:8a:69:a0:7a:71:aa:4c:d6:fb:b0:d9:
                    58:8f:f2:f9:e1:c9:9d:54:3d:82:60:81:b2:59:c5:
                    6a:c7:ff:69:c0:f4:31:08:a1:61:da:62:35:82:d5:
                    63:7a:af:4b:66:9e:73:23:63:e8:de:30:74:c4:ed:
                    e0:31:5f:66:70:66:27:fe:8e:a3:4f:c3:98:66:fc:
                    af:2c:0a:7d:f6:ce:e9:26:48:be:e7:12:7b:09:56:
                    7a:9a:f8:bc:9f:6d:5e:c1:56:a9:1b:70:cd:01:71:
                    87:a4:49:d1:3c:5b:31:bd:9d:77:4c:fe:7f:03:2d:
                    44:b9
                Exponent: 65537 (0x10001)
        X509v3 extensions:
            X509v3 Key Usage: critical
                Digital Signature, Key Encipherment
            X509v3 Extended Key Usage: 
                TLS Web Server Authentication, TLS Web Client Authentication
            X509v3 Basic Constraints: critical
                CA:FALSE
            X509v3 Subject Key Identifier: 
                9C:66:DB:15:43:B5:06:86:B1:00:76:AC:9E:8F:5E:C0:3C:29:87:E8
            X509v3 Authority Key Identifier: 
                keyid:14:2E:B3:17:B7:58:56:CB:AE:50:09:40:E6:1F:AF:9D:8B:14:C2:C6

            Authority Information Access: 
                OCSP - URI:http://r3.o.lencr.org
                CA Issuers - URI:http://r3.i.lencr.org/

            X509v3 Subject Alternative Name: 
                DNS:*.askubuntu.com, DNS:*.blogoverflow.com, DNS:*.mathoverflow.net, DNS:*.meta.stackexchange.com, DNS:*.meta.stackoverflow.com, DNS:*.serverfault.com, DNS:*.sstatic.net, DNS:*.stackexchange.com, DNS:*.stackoverflow.com, DNS:*.stackoverflow.email, DNS:*.superuser.com, DNS:askubuntu.com, DNS:blogoverflow.com, DNS:mathoverflow.net, DNS:openid.stackauth.com, DNS:serverfault.com, DNS:sstatic.net, DNS:stackapps.com, DNS:stackauth.com, DNS:stackexchange.com, DNS:stackoverflow.blog, DNS:stackoverflow.com, DNS:stackoverflow.email, DNS:stacksnippets.net, DNS:superuser.com
            X509v3 Certificate Policies: 
                Policy: 2.23.140.1.2.1
                Policy: 1.3.6.1.4.1.44947.1.1.1
                  CPS: http://cps.letsencrypt.org

            1.3.6.1.4.1.11129.2.4.2: 
                ......v.D.e......@....(.......1.?.3........yu.Y5.....G0E.!....Q...g^Mrv..."gK
..Y|.q.....|.. d?...#...<...k..V.]J.._.&........v.}>.....Uh$....R.y+..x...j.h.~".....yu.Y].....G0E. ~<H..../Z1..%k.;.Q$$3....m...q)..!....w...O.!............J.C.He...L
    Signature Algorithm: sha256WithRSAEncryption
         18:05:01:11:f4:4e:e7:51:c4:5e:91:9e:3f:ec:bd:8b:32:85:
         5d:74:08:f7:98:15:74:7b:9f:80:49:79:64:a8:ea:7b:0a:0c:
         25:ed:08:d0:09:23:e2:48:55:ca:10:4e:d6:d9:d4:34:c3:85:
         9b:2b:d0:c4:22:b0:d4:66:bf:49:3a:6d:7e:78:b3:e6:56:c3:
         18:83:f4:31:0e:62:3f:34:a6:9d:c4:82:cd:45:13:60:2d:ca:
         9e:7f:5c:63:f7:e4:49:8d:1b:a2:75:cd:72:97:fb:2a:c0:c7:
         62:76:46:93:5f:8c:84:a4:42:99:50:f6:ef:aa:a6:f7:ab:41:
         91:5c:7a:9e:b6:59:4b:e5:b2:da:47:7e:30:30:56:6d:84:0c:
         aa:7e:14:80:6f:31:4e:6f:fa:84:d4:42:0c:ab:b8:8c:c6:77:
         b1:96:e8:a1:e2:ba:e3:57:e1:f8:b4:b9:40:52:e1:da:62:4c:
         d3:0c:7e:41:05:75:a6:ab:9b:71:54:07:36:93:83:f8:f3:38:
         ba:b3:41:eb:32:64:39:74:62:6a:d0:18:6d:f7:72:5a:3a:d5:
         08:e9:29:b0:e3:44:95:ac:2e:d7:29:b6:7a:3c:df:ad:08:55:
         1a:e3:ce:c7:57:fe:8f:1f:66:25:ba:59:73:83:e1:53:ac:38:
         a2:4d:36:39