This link from the API docs sums it up pretty well. Looks like it returns the second.
Also. To fix your issue, os.date returns strings. You can see this by typing print(type(os.date("%S"))); into the command bar. Your refactored code should look like this:
while task.wait(1) do
print(os.date("%S"))
if os.date("%S") == "10" then
print("10 Seconds")
end
end