inline FLEX_ALWAYS_INLINE_FLATTEN flex_float operator()(flex_date_time dt) const { 
   return dt.microsecond_res_timestamp();
 }
 inline FLEX_ALWAYS_INLINE_FLATTEN bool operator()(const flex_float t, const flex_date_time u) const { return t < u.microsecond_res_timestamp(); }
 inline FLEX_ALWAYS_INLINE_FLATTEN bool operator()(const flex_float t, const flex_date_time u) const { 
   return std::fabs(t - u.microsecond_res_timestamp()) < flex_date_time::MICROSECOND_EPSILON; 
 }