Find earliest time form the list.

Same as Earliest, but accepts null.

IsBetween true if `t` is between `a` and `b` (inclusive).

IsBetweenOptional true if `t` is (1) between `a` and `b`, (2) `a` < `t` and `b` is zero, (3) `t` < `b` and `a` is zero, and (4) both `a` and `b` is zero.

Same as Latest, but accepts nil.