Siemens Hicom 100e (112-118) Series

At System Manager Extension (200 or 11 usually)
1.- Dial *94
2.- Press Tick to enter
3.- Scroll to TIME & tick
4.- Tick to alter time
5.- Dial In The Time ie HHMM
6.- Press Tick & lift handset & release


Siemens Hicom Office Series

 

At System Manager Extension (201 or 21 usually)
1.- Dial *95
2.- Display reads "Enter User "
3.- Dial *95 & tick
4.- Scroll to TIME & tick
5.- Tick to alter time
6.- Dial In The Time ie HHMM
7.- Press Tick & lift handset & release