Insert Slide Number, Date and Time
If you click Insert > Slide Number, or Insert > Date and Time, PowerPoint will bring up the Header and Footer dialog box. Alternately, clicking View > Header and Footer will also bring up this dialog box. So what does the Insert > Slide Number and Insert > Date and Time actually do? They are there for another reason, somewhat harder to discover.
First, click on a textbox and go into editing mode. To go into editing mode, you can press the F2 key on your keyboard. Next, click Insert > Slide Number. This will allow you to insert the current slide number onto the textbox.
Finally, try clicking Insert > Date and Time. This time round, a Date and Time dialog box will appear, with different formats available for you to choose. You can have it update automatically, changing the language, calendar type, and also setting the defaults.