Dates and times are entered in your local date and time format. These formats are controlled by the International System Preference pane.
Apple Menu > System Preferences… > International
You can also enter the date and time in a number of more friendly formats. As you type your date or time, look for the computer's interpretation below the text field.
Below are some examples of acceptable formats for UK dates.
The event must not be in the past or within the next two minutes.