If you are using one of the phone or tablet apps, My Study Life will automatically match the date and time format used by your device.
If you are using the web app you can configure this from the Locale screen under Settings. There are two ways to configure the time and date format. You can either toggle the 12 or 24 hour option, or using the experimental culture setting. The culture setting will override the 12 or 24 hour setting and will also change the date format (as it is an experimental feature, not all cultures are currently supported).