std::chrono::time_point::operator+, operator-
From cppreference.com
                    
                                        
                    < cpp | chrono | time point
                    
                                                            
                    |   time_point& operator+=( const duration& d );  | 
(1) | (since C++11) | 
|   time_point& operator-=( const duration& d );  | 
(2) | (since C++11) | 
Modifies the time point by the given duration.
1) Applies the offset 
d to pt. Effectively, d is added to the internally stored duration d_ as d_ += d.2) Applies the offset 
d to pt in negative direction. Effectively, d is subtracted from internally stored duration d_ as d_ -= d.Contents | 
[edit] Parameters
| d | - | a time offset to apply | 
[edit] Return value
*this.
[edit] Exceptions
(none)
[edit] See also
|    modifies the time point by the given duration  (function template)  | |