Calls to undefined functions module_exist() and date_formats()

I'm using Drupal 5.1 with CCK 5.x-1.3. Just upgraded to Date 5.x-1.3 and I'm seeing calls to undefined functions.

The first was:

[01-Mar-2007 09:48:56] PHP Fatal error: Call to undefined function: module_exist() in /Library/WebServer/Documents/intranet-beta/sites/all/modules/date/date.module on line 22

Not being an expert on the code or the API's, I'm *assuming* (and you know what they say about that...)
that you meant module_existS(). If I make that change, I get as far as line 790...

[01-Mar-2007 09:56:52] PHP Fatal error: Call to undefined function: date_formats() in /Library/WebServer/Documents/intranet-beta/sites/all/modules/date/date.module on line 790

At this point, I officially declare I am in over my head, and have missed something....

Oh, by the way: Mac OS X 10.3.9, PHP 4.3.11, www.entropy.ch Release 1.

Thanks.....