When the date and Time field is used in a formula, it may display as the Date field, not as the Date & Time field. And when it is converted to the Date & Time field it will be showing the 12hr format.
To get the 24hr format for a time, the below formula can be used (added 24 with HH and changed MM to MI).
TO_CHAR({systemnotes.date},’MM/DD/YYYY HH24:MI:SS’)
Instead of ({systemnotes.date}, use the required date field.