calendarmacos
Since upgrading to Yosemite, I can't enter a New Event in Calendar using the 24 hour clock. If I try to enter the start or end time as 14:00, it enters the 1 and then beeps at me.
Calendar does display events in 24 hour format, I just can't create a new event using it. I've check the calendar and Mac settings and they all appear correct.
The solution is actually simpler than that. In Calendar preferences, click "Default Calendar" and scroll to the bottom and select "Selected Calendar." Now, whichever calendar you have highlighted in the sidebar becomes the calendar in which you are working.
If you have a number of events you want to enter and you have their times in another zone, it's very handy to use Calendar's time zone support.
First, go to Calendar->Preferences, and click Turn on time zone support.
This will enable a dropdown in the titlebar of your Calendar window:
You can then use this menu to change the display of everything in your calendar to a new time zone. Pull it down and select Eastern time (or whatever other time zone or city your event is in), and Calendar will convert all your times to that zone.
At this point, you can start entering events, using the times listed in the program for the event. You will also see a new detail in the event detail popup:
This shows what time zone the event is in.
Once you've entered all the events, turn the time zone back. That will display all your events in your own time zone. If you are syncing with your phone (which knows what time zone it is in) all the events will remind you at the right time.
Best Answer
I do not have Yosemite but on Mavericks you can set it up as follows:
Open system preferences Languages and Regions
Click on the 24 Hour time format.
That should change the time format in Calender.
I can enter any time (as 2 digits)
Let me know if it works in Yosemite.