I use Miami myself, so can't suggest much other than reading the AmiTCP/Genesis
docs to make sure you're entering time update settings correctly.
An alternative is to use a time sychroniser like FACTS from Aminet.
You can set it to run (as a hidden commodity) when Genesis goes online.
If you have a system clock speed problem caused by a GFX card like I do, you can
set FACTS to auto-update every set number of minutes.