PMBOK Guide Page 224 defines it as
The amount by which the project is ahead or behind the planned delivery date, at a given point in time, expressed as the difference between the earned value and the planned value.
Practice Standard for Earned Value Management (by PMI) defines it as
The schedule variance (SV) determines whether a project is ahead of or behind schedule in accomplishing work.
It further says...
Schedule variance (SV) is very often misinterpreted as a time based indicator , for example are we early or late? it is not, rather its an indication of the physical status
Which one do you think is more accurate ?