User to input an hour time period with an option of 24 hour or 12 hour type representation

I am trying to think of the best way for a user to input a start hour an end hour but I need to be able to support both AM/PM and 24 hour time representation. Currently I was thinking of a drop down list with AM, PM, and Military to distingui开发者_如何转开发sh each time but since this is a page for time sheets that would mean 56 drop down lists and that sounds TERRIBLE

Do you have suggestions on how to do this ? Is there a javascript time picker that already does this ?

There's a box where you can type the hour and grayed-out AM/PM dropdown/radio button set. If the user types in a number less than 13 in both boxes, the AM/PM chooser activates and is required. If they enter a number greater than 12 in either box, it doesn't.