>>In SQL Server unknow dates are usualy represented by NULLs. In MS SQL Server the minimum date that can be stored in the datetime data type is January 1, 1753
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>Is there something that can be done in our case to support those previous century dates other than to apply a design of additional fields to hold numeric values of those equivalent dates and to apply some kind of logic to extrapolate the related values where required?

Depending on how you use those dates, you can store them either as numbers or as strings.