Question:

Under My Schedule Calendar the hours of a day can be displayed:

1. Using the 12-hour clock, the AM - PM time format displays: 
2. Using the 24-hour clock, not the display:

What controls if My Schedule Calendar is displayed with a 12-hour (AM/PM) or 24-hour clock? 

Answer:

The display is based on the customer's Locale. Salesforce determines the display for each supported Locale. This is a hard-coded value by Salesforce.

E.g. Locale English (United States) displays the 12-hour clock and Locale French (France) displays the 24-hour clock.

A customer's Locale can be set following the steps below:

1. Open the customer's User Detail page.
2. Click on Edit.
3. Scroll down to the Locale field.
4. Set a value and click on Save.

Related Documentation:

Salesforce Help Documentation: Supported Date and Time Formats (ICU)

CRM Help Documentation: Managing Scheduled Calls and Activities

Thank you