A contract start date that has already passed now tells you

When you generate a contract, the start date is filled in from what the request originally stated. On a request that has been open a while, that date has often passed — and nothing said so.

Now a start date in the past carries a warning under the field, on both the contract step and Move to signed. The date itself is left exactly as the request stated it and you can still choose a past date deliberately; the warning is only there so a stale date is never sent without someone having looked at it.