Just a guess here but the field is a datetime field which is not nullable, therefore when you save a record with no value it will return the default minimum value. The figure you see in Excel should be -53690, which if you run
select cast(-53690 as datetime)
returns a value of 1753-01-01 00:00:000, I would suggest putting an IF statement in the field which checks the value and returns blank if it is this value, otherwise it returns the stored value.
Hope that makes some sort of sense, falling asleep rapidly