Aastra 57i IP Phone User Manual page 118

Aastra Web UI
1. Click on Basic Settings->Preferences->Time and Date Setting.
2. In the "Time Format" field, select the time format you want to use on
your phone. Valid values are:
12h (12 hour format) (default)
24h (24 hour format)
Note: The time displays on the phone's idle screen in the format you select for this
field.
110 Model 57i IP Phone User Guide