That's because it is now a serial number. 23:30 is the equivalent of the fraction of the day 0.98.
Also, are you wanting only the 23:30, and not the date with it? If so, then format as Time rather than Date, and choose the hh:mm option.
And if you have an extra column, you can use this formula to get what you want: Assume that your date is in cell B3, then in C3, put this:
=HOUR(B3)&":"&TEXT(MINUTE(B3),"00")