Yes, local datetimes shouldn't be used to represent instants. But I think you can still add days or hours to them.

Don't think of the hours here as units of time, just as parts of the datetime. It's the same as with months: A month doesn't have a defined length in seconds, but you can still add X months to a date without worry, because you aren't using them as units of time, but as "units of calendar". Duration vs Period in java.time parlance.