I think you're asking for an option to display times using a 24-hour clock, rather than as 12-hour clock. If that's the case, then you do that by updating your Locale preference and selecting "English (Great Britain)" or "English (Ireland)". Please note that your Locale preference determines not only how times are displayed, but also dates and numbers. So if you change your Locale preference to "English (Great Britain)" or "English (Ireland)", then dates will also be shown as dd-mm-yyyy. For example, 12-09-2013 or 12 Sept 13 means Sept 9, 2013.
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