Validating Time entries

Hi I have had a search through the forum but can't seem to find an answer to this ...

Is there a way to "validate" the way the time is put in cells on my spread sheet.

for example If i put in 23:00 or 08:45 that is fine but if i put in 2300 or 0845 the cell returns a value of 00:00. The cells are formatted for the time hh:mm

Many thanks for any help you can give

Paul

Brillient thanks very much Dave for the prompt response

Paul