How can I switch between 12 and 24 hour clock?

The app will display either 12 or 24 hour clock (also known as military time) depending on your device's settings. Specifically, this option is controlled by going to Settings on the device (iOS Settings, not the app's own Settings screen), then choosing General/Date & Time.

After changing this setting, you may need to scroll the timeline a little for the reformatted times to appear.

This setting also controls the format of times displayed the Day and Year Events screens.