file descriptor referring to the timer
- it_value: Returns the amount of time until the timer will next expire. If both fields are zero, then the timer is currently disarmed. Contains always a relative value, regardless of whether the TFD_TIMER_ABSTIME flag was specified when setting the timer. - it_interval: Returns the interval of the timer. If both fields are zero, then the timer is set to expire just once, at the time specified by it_value.
0 on success or -1 on error. Sets errno in case of error.
Returns the next expiration time of fd.