Having a keep alive max of only 18h 12m and 15s can be a real limit for constrained IoT device that for instance needs to send a single message per day. A possible solution is to create a new CONNECT flags byte (anyway required to support security). So the current flag on bit 7 instead of reserved should be:

• 0=no additional flags
• 1= another byte of flags with the following structure
  • Reserved (7)
  • Reserved (6)
  • Reserved (5)
  • Reserved (4)
  • Reserved (3)
  • Reserved (2)
  • Reserved (1)
  • KeepAliveUOM
    • 0 = seconds
    • 1 = minutes