1. In PowerPoint for the web, click Open in PowerPoint.
2. On the Insert tab, click Header & Footer.
3. Enter the slide number or date and time information in the Header and Footer box, and click Apply to all.
4. Click Save.

To add slide numbers or the date and time, use the PowerPoint desktop application.
Click Open in PowerPoint in PowerPoint for the web.
Click Header & Footer on the Insert tab.
In the Header and Footer box, enter the slide number or date and time information, and then click Apply to all.
Save the file.