The Closing Date is the last day you can complete your interview on.
You must complete the interview before 23.59pm GMT on the day your interview is set to close. So if your closing date is 23 February, you have until 23.59 GMT (Greenwich Mean Time) on that day to complete your interview by.
If you are not in the GMT zone, you can use this site -
wwp.greenwichmeantime.com - to help you find the time difference between where you are and GMT.
If you fail to complete the interview before this time the interview will show as 'Expired'. You will no longer have access to the interview and you will have to contact the interviewer directly to ask about extending the deadline for you.
You can contact the interviewer by replying to your invitation email and they will then respond to you directly.