That's not good form. Neither the Client nor the Research Organisation should jump out of the starting gate before they have signed, sealed and delivered the contract. I would suggest that you avoid making the start date the date of signature of the contact. There should be an interval between the contract signing and the start date. A Client can obtain a slight tactical advantage over the Research Organisation in proposing a deferred start date.
Most research projects require a certain amount of prep time before official commencement.
The Research Organisation must buy consumables and equipment. It must organise its personnel. Staff may need to be freed up from other work-related activities. The Research Organisation must make sure that the necessary facilities are available. If the start date is the same as the contract signing date, Research Organisations have a tendency to find reasons to charge their Clients for this time even though not much in the way of research is going on.