How to enter custom format for both current date and time in cell?

I know we can enter a date and time into a cell separately. But I'd like to create a custom format that allows me to enter both the date and time stamp concurrently in one cell. However when I select the custom format and create such a format it enters the date correctly but not the time. The time defaults to 12:00 PM even though I'm asking for the custom format to time stamp the current time into that cell using the custom format.

What am I doing wrong? Is there a way to get a custom formatted cell to input both time and date stamp correctly simultaneously in the same cell or do I have to create a separate cell for the time stamp? Thanks in advance.

You can enter the date and time in a cell, as, for example 10/08/2011 14:41

Then you can format that however you like, e.g.,

hh:mm dd mmm yyyy

yyyy-mmdd hh:mm

mm/dd/yyyy hh:mm

But how do I enter both the time and date in the same cell? Is there a way to do this? In other words I pre-format the cell using the custom format and then I enter the date and it automatically enters a time stamp. When I do this it always time stamps it using the default time of 12:00 PM. But that's not right because Excel should be picking up the serial number based on the internal clock of my computer and time stamping the exact hour/minute/second in that cell.

But how do I enter both the time and date in the same cell?

I don't understand the question; I gave you an example in the first sentence of my post.

You can also enter the current date and time by selecting a cell and doing Ctrl+; space Ctrl+:

The Ctrl+; enters the date and the Ctrl+: the time.

and then I enter the date and it automatically enters a time stamp.

What automatically enters a time stamp?

If you chose to do that, you'd want to add the values to get a serial date/time value, not catenate them to create text that looks like a date/time.