Windows by default uses a 12 hour time format to display the time on the taskbar. For those who want to change it to 24 hour time format, you will find that there is no option for you to do so in the Right click->Adjust Date/Time menu.
To change the time to 24 hour time format, this is what you have to do:
Go to Control Panel (Start->Control Panel) and select Regional and Language Option.
Under the Regional Options tab, click on the Customize button, follow by the Time tab.
Under the Time format field, click on the dropdown box and select “H:mm:ss”. If you want it to display leading zero, select “HH:mm:ss”.
Click OK to close all the windows. That’s it. Your clock should be in 24 hour time format now.