Yes, it is possible to fill only half of a cell in Microsoft Excel. Here's how you can do it:

1. Select the cell that you want to fill half of.
2. Click on the "Borders" button in the "Font" group on the "Home" tab.
3. Select the "More Borders" option at the bottom of the drop-down menu.
4. In the "Format Cells" dialog box that appears, select the "Border" tab.
5. Under "Presets," select the border style that cuts the cell in half (e.g., "Diagonal Down").
6. Click on the "Fill" tab.
7. Under "Fill Effects," select the "Gradient" option.
8. Choose the "Two Colors" option.
9. Select the color you want for the top half of the cell under "Color 1."
10. Select the color you want for the bottom half of the cell under "Color 2."
11. Adjust the "Shading Styles" and "Variants" options as desired.
12. Click "OK" to apply the formatting to the cell.

To make a half cell equal to half an hour, you can adjust the column width to make it half the width of a full hour cell. To do this, simply click and drag the right border of the cell to the left until it is half the width of a full hour cell. Now, when you enter a value of into the half cell, it will represent half an hour.