Refactor rate limiter and datetime formatting

src/ddns_service/email.py

@@ -2,6 +2,8 @@
 
 import logging
 import smtplib
+
+from . import datetime_str
 from email.mime.text import MIMEText
 
 
@@ -91,11 +93,11 @@ Hostname: {hostname}
 """
         if last_ipv4:
             ip = last_ipv4[0]
-            last_update = last_ipv4[1].strftime("%Y-%m-%d %H:%M:%S")
+            last_update = datetime_str(last_ipv4[1])
             body += f"IPv4 address: {ip} (last update: {last_update})\n"
         if last_ipv6:
             ip = last_ipv6[0]
-            last_update = last_ipv6[1].strftime("%Y-%m-%d %H:%M:%S")
+            last_update = datetime_str(last_ipv6[1])
             body += f"IPv6 address: {ip} (last update: {last_update})\n"
         body += f"""Expiry TTL: {expiry_ttl} seconds