I have changed the regional settings in sharepoint online, but this does not reflect the in the fields. The field datetime is still showing am/pm formatting. I would like to change the field to 24 hour formatting.
Change the format of the datetime field to 24 hour format.
Per my test, even if you set time format to 24 hour in the regional settings, it automatically changes the time format of date and time column created by yourself, but it cannot automatically change the time format of default site columns, such as start time.
As a workaround, you should go to Site settings -> Site Columns -> Start Date -> Set Date and Time Format as Date & Time -> Set Update all list columns based on this site column? as Yes. Then go to the list, delete the original Start time column, re-add the Start time column.
