Most cameras don't let you customize the timestamp overlay that much.
If you're using
Blue Iris's overlay, go to Camera Properties > Video tab. Near the bottom there is an "Enable overlays" checkbox and an "Edit…" button just to the right of that. Click Edit. A new panel should appear, showing a snapshot with your overlay in it. You can drag it around and resize it here. Right-click on the timestamp and a panel should appear allowing you to modify the text, font, color, background, etc.