Added target docs on the clock_... functions.

doc/cbm610.sgml

@@ -224,6 +224,13 @@ No mouse drivers are currently available for the Commodore 610.
 <sect>Limitations<label id="limitations"><p>
 
 
+<sect1>Realtime clock<p>
+
+The realtime clock functions use the CIA1 TOD clock. As that clock only stores
+the time but not the date, the date set by <tt/clock_settime()/ ist simply stored
+inside the C library for retrieval in the same program via <tt/clock_gettime()/.
+
+
 <sect1>Kernal and hardware access<p>
 
 Since the program runs in bank 1, and the kernal and all I/O chips are located