First you have to open System Preferences/ International and choose the Formats tab. If you click customize at Times you will find several formats of time that can be customized: short, medium, long, full. What is shown in the menubar is the medium format, so we gonna have to choose that from the drop-down menu. Here you can simply drag and drop the Time elements from below to create your custom time format.
You will soon discover that date and day of the week is not among the options. The trick is that you need to copy the required fields from the Dates window.
You need to close the Time window for now and open the Dates customize window and choose any format that contains the type of display for date and time that you require. Now, simply select and copy the required part from the sample line. Close the window and go back to Time customize and paste in the code from the clipboard to the appropriate place. You can also add any punctuation marks and spaces to fine tune the look.