Multi-valued columns in a relational database is never a good idea.
The recommended way is to have a separate table with a datetime field and a reference field to the record in the original table it belongs to.
CREATE TABLE ORIGINAL (
OriginalId int NOT NULL IDENTITY PRIMARY KEY
,
)
CREATE TABLE YOURDATETIMES (
YourDateTimeId int NOT NULL IDENTITY PRIMARY KEY
,OriginalId int NOT NULL REFERENCES Original.OriginalId
,Value datetime2 NOT NULL
)