Use "es la una" for one o clock.
Use "son las" for everything else.
Since this is 8:20, you would use son las.
- 8 = ocho
- and = y
- 20 = veinte
The way to say 8:20 in Spanish is:
Son las ocho y veinte.
You did not give am or pm, but am would have "de la manana" (accent above first n) after the veinte, and pm would have "de la noche" (since 8:20pm is night time) after veinte.