If you want to bypass validation it probably means that your validation logic is not what you need.
For instance "Start Date shouldn't preceed today" only makes sense when the record is created. Of course, old records may and will have an older Start Date. Maybe your validation rule needs to be modified to use the ISNEW () operator so that it only runs when the record is created.
AND (ISNEW(), <your existing validation here>)
Another way would be to conditionally disable your rule depending on a custom settings object, which you could disable manually or in your trigger. But that is overthinking it and making it even more complicated. I think that the above approach may be the best for you.