This command has been moved to the new "Time Formats" window in DP3.1

From page 15 of the DP3.1 update notes:

Time Format window

The new Time Format window (Basics menu) lets you specify the
time format to be displayed throughout Digital Performer in
event lists, time rulers, dialogs, windows and any place where
time is displayed. This feature allows you to view SMPTE time
code or real time throughout Digital Performer, even in places
where these time formats were not previously available, such as
the Edit Grid in the Graphic Editor and Sequence Editor.
Figure 37: Time Format window.


Customizing the time format display

Click the Details triangle to further customize which time formats
are displayed in various areas of Digital Performer. You can
specify certain time formats as needed. Edit window mini-menu
time format settings have been consolidated in this area of the
Time Format window, as have time ruler settings and other time-related
settings. Notice, for example, that the setting for the
number of ticks per quarter note, previously in the Data Display
Preferences (File menu), has been moved to this window.