At least in Java, storing a date/time as a primitive improves immutability and is a good pattern.

You don't expose the underlying long value publicly so you end up with something like the following:

private long primitiveDate;

public Date getDate() { return new Date(primitiveDate); }

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This prevents calling methods from doing something stupid like:

o.getDate().setTime(123455L)

to modify a date that shouldn't be editable.