I assume that your model number is 28112EE2 (there is no 2811ee_2). If that's the case then here is how you do it:
1. Make sure the answering system is ON.
2. Press and hold the DAY/CHECK button to set the day of the week.
3. Press and hold the HOUR button to set the hour (a.m. or p.m.). Example: 12AM, 1AM, or 12PM, 1PM.
4. Press and release the MIN button to advance the clock in one minute intervals. Press and hold to increase by 5-minute increments.
5. After the time is set, the real time clock will be displayed on the handset within 1 minute.
NOTE: You must set the day manually. You may choose to set the time manually as well, although the time is automatically transmitted by your local phone company as part of Caller ID service. If you subscribe to Caller ID service, the current time is set automatically when you receive your first CID record.
In case you need a manual for later use (or I was wrong regarding the model number), you can find the right one here
If you have any more questions, come back here and leave a comment, I'm happy to help.