diff --git a/main.py b/main.py
index d2335f5..bc95feb 100644
--- a/main.py
+++ b/main.py
@@ -7,6 +7,8 @@ logging.basicConfig(
 import asyncio
 import aiohttp
 import aiomqtt
+import ssl
+from datetime import datetime, timedelta
 
 
 HOSTS = {
@@ -25,9 +27,67 @@ HOSTS = {
 def alert_tanner(msg):
     pass
 
-def main():
-    pass
+async def check_host_cert(host, port):
+    "check a single host's cert"
+    try:
+        # default context does hostname checking and certificate validation
+        ssl_context = ssl.create_default_context()
+        # open_connection does the TCP connection and TLS handshake
+        _reader, writer = await asyncio.wait_for(asyncio.open_connection(
+            host, port, ssl=ssl_context), timeout=10)
+
+        cert = writer.get_extra_info('peercert')
+        writer.close()
+        await writer.wait_closed()
+
+        if not cert:
+            # this case should be rare if handshake succeeded
+            msg = f"Could not get certificate for {host}:{port}"
+            logging.warning(msg)
+            alert_tanner(msg)
+            return
+
+        expiry_date_str = cert['notAfter']
+        expiry_date = datetime.strptime(expiry_date_str, '%b %d %H:%M:%S %Y %Z')
+
+        time_left = expiry_date - datetime.utcnow()
+
+        if time_left < timedelta(days=7):
+            msg = f"Certificate for {host}:{port} expires in less than a week: {expiry_date}"
+            logging.warning(msg)
+            alert_tanner(msg)
+        else:
+            logging.info(f"Certificate for {host}:{port} is valid until {expiry_date} ({time_left.days} days left)")
+
+    except ssl.SSLCertVerificationError as e:
+        msg = f"Certificate verification error for {host}:{port}: {e.reason}"
+        logging.error(msg)
+        alert_tanner(msg)
+    except ssl.SSLError as e:
+        msg = f"SSL error for {host}:{port}: {e}"
+        logging.error(msg)
+        alert_tanner(msg)
+    except (asyncio.TimeoutError, OSError) as e:
+        # Per instructions: log and move on for connection errors
+        logging.error(f"Connection error for {host}:{port}: {e}")
+    except Exception as e:
+        # Catchall for other things
+        msg = f"An unexpected error occurred for {host}:{port}: {e}"
+        logging.error(msg)
+        alert_tanner(msg)
+
+
+async def main():
+    tasks = []
+    for host in HOSTS['http']:
+        tasks.append(check_host_cert(host, 443))
+
+    for host in HOSTS['mqtt']:
+        # standard port for MQTTS is 8883
+        tasks.append(check_host_cert(host, 8883))
+
+    await asyncio.gather(*tasks)
 
 
 if __name__ == '__main__':
-    main()
+    asyncio.run(main())