Go to workspace settings
In Slack, type the command "/events settings" - you'll see the following buttons:
Click "Change Date/Time Format" to specify your options for date and time formatting. The default installation is set to 12-hour time and the Month / Day / Year format. Once this button is clicked, you'll see two drop down fields (shown below) to change these settings for the workspace.
Changes you should see
Once changed, these setting take effect immediately. Changing the format only changes how events are input. When the event is viewed in a channel, it will default to your Slack preferences.
Note: Slack is having some trouble displaying 24 hour time properly. This is platform wide, however with this Eventbot change you'll be able to input and edit a 24 hour formatting time.