The old (original) ZDay was designed and programmed by Braveheart (ZDay founder). It was used to show roughly what the game could look like. It was made using the Blender Game Engine and Python. This engine is no longer in use in the official ZDay. The game had many bugs. Here is a list of some:
1) Zombies were not synced across the server. This means that when one player sees a zombie another would not.
2) There were no animations.
3) Only one type of gun.
4) You could be thrown out of the map after spawning.
The first time this version was released people took it as the real one. Because of this they thought the game sucked and said so to the devolpers. Braveheart decided to turn off the servers for it to avoid confusion. The verson was again released after questions were raised about it and if they could play. Braveheart decided to update it. He had less free time than he thought and he could not turn on the servers much. At this time there is no Old ZDay that can be played.