What date should be treated as the date of contract if a digitally signed agreement is drawn up for a house or other real estate?

The date of contract is when the agreement is finalised. This means the moment when the digital signature of the last person who is expected to sign the deed has done so. You need to write that date into the transfer tax return's line “date when the deed of sale or other contract is signed”.

Because the Property Transaction Service creates an electronic application for registration automatically, and because the deadline for transfer tax filing and payment is the time when registration is requested, the buyer needs to both have filed the transfer tax return and paid the tax by the date of contract, i.e. the time when the agreement is finalised.