It is probably possible if the Key field of the Tariff table were constructed with a compound hyphenated key like [Related Supplier] - [End Date].
ie it would need to be a key field that could be calculated. so that the record entered with a date of January 1 would be able to do that subtraction of one day and construct the link back to the proper parent. The form would need to calculate the Key field as the Key field of a table may not be a formula field. So the form would need to calculate it.
Can you tell me what uniquely , in theory, defines a Tariff - ie is it that combination of Supplier and effective dates? Do you even know the end date at the time a new tariff record is created? Or do you only know the end date when you are about to have a new Tariff rate come into effect.
