I've found that different people have different expectations, and that can cause confusion.
But generally, for a dinner party, a time is more specific, and for somethign more casual like a BBQ, the invite might be "swing on by Saturday afternoon" (and anytime that afternoon really is fine).
If in doubt, perhaps risk manage by inviting people to arrive before dinner is perfectly timed to come out, (I think that's why appetisers were invented) and considering what you know about the invitees puncutality when giving them a time (sometimes you can invite people who are always late to a time 1/2 hour earlier than everyone else).
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But generally, for a dinner party, a time is more specific, and for somethign more casual like a BBQ, the invite might be "swing on by Saturday afternoon" (and anytime that afternoon really is fine).
If in doubt, perhaps risk manage by inviting people to arrive before dinner is perfectly timed to come out, (I think that's why appetisers were invented) and considering what you know about the invitees puncutality when giving them a time (sometimes you can invite people who are always late to a time 1/2 hour earlier than everyone else).