Atari ST series Technical Reference Manual page 270

Tsettime()
Allows you to set the GEMDOS time.
C macro format
int time;
Tsettime(time);
Machine language format
move.w #time, - (sp)
move.w #$2D, - (sp)
trap #1
addq.l #4,sp
Inputs
time word
Interpretation of GEMDOS
Bit Number Description
0-4 Seconds divided by 2
5-10 Minutes
11-15 Hours
Results
None
See also
Tsettime(), Tgetdate(), Tsetdate()
262
Tsettime
Set GEMDOS Time
A 16-bit code that indicates the time.
time:
Range
0-29
0-59
0-23
Opcode = 45 ($2D)