Changing Time on a Honeywell Vista 15, 20, 21,etc
To CHANGE the system TIME/DATE
1. Enter your 4-digit MASTER code
2. Press [#] then  then 
3. Press [*] when the time/date is displayed. A cursor will appear in the first digit of the hour (To move ahead, press [*], backwards [#]
4. Enter the 2 Digit hours setting, 2 digit minute setting. Press  for PM or  for AM. Enter the last two digits of the current year. Enter the 2-digit month, Enter the
5. To exit, press [*] when the cursor is at the last digit or, or wait 30 seconds.
Full PDF manual here http://www.security.honeywell.com/documents/vista15UM.pdf