Expression.Constant(Convert.ChangeType(value, Nullable.GetUnderlyingType(memberAccess.Type)));.

Expression.Equal(memberAccess, filter);

ConstantExpression test = Expression.Constant(Nullable.GetUnderlyingType(memberAccess.Type)),

filter = Expression.Constant(
    Convert.ChangeType(value, memberAccess.Type.GetGenericArguments()[0]));

Expression typeFilter = Expression.Convert(filter, memberAccess.Type);

Condition(
    Equals(yourStringExpression, null),
    Constant(null, typeof(DateTime?)),
    Convert(
        Call(DateTime.ParseExact, yourStringExpression, ...),
        typeof(DateTime?)
    )
)