University departments advertising faculty positions usually take great care to mention all the relevant details of the position in the ad, to the extent that such details can be specified. There is often a process to get the ad cleared by various layers of the campus bureaucracy (for example, at my university the ad is part of the search plan that has to be approved by an associate dean). So it's a pretty safe bet that the ad has been scrutinized by several people (hopefully competent ones) and that the omission of the start date is not accidental.
My conclusion is that the start date is almost certainly not specified because it cannot be specified. I can think of two possible scenarios where that would be the case:
The department advertising the position is not sure when the start date would be. E.g., maybe they are waiting for funding to materialize and are not sure when the funds would be approved, or when some other approval process outside the department would terminate, but they are already confident enough that this will happen at some reasonably near future time to start the recruitment process.
The department is flexible about the start date, i.e., maybe they would be happy with having the new faculty member start either in the spring or in the fall, and are leaving those details to be discussed/negotiated later during the interview and negotiation stages of the recruitment.
My advice is not to assume anything that isn't explicitly written in the ad. You can always just apply for the position and see what happens. Or, if it's important for you to know the start date to decide whether to apply for the position, you should contact the institution and ask about it. But it wouldn't surprise me if they reply with a vague answer saying they can't be sure, since as I explained above, probably if they had a specific date in mind they would already have mentioned it in the ad. Good luck!