I do not know in your country, but in Spain following the indications in this regard of the WHO and the current Health CCs and, in particular of Neuroscience, the "official diagnosis", if it is indicated or required to specify it -for example, to extract organs for transplantation- it is established by means of a totally flat EEG for at least three minutes, confirmed (according to the means that are possessed) by another Health Professional, EVEN THOUGH THE HEART, FOR WHAT IT IS, KEEPS BEATING.

https://www.health.ny.gov/professionals/hospital_administrator/letters/2011/brain_death_guidelines.htm

Nicely penned article similar to the index topic.

Could it be after it is decided the heart cannot be restarted - after the person cannot resuscitated? In that case, brain activity would be moot.

These are two different things:

1) confirmation of brain death (e.g. in the case of transplantation) and then the actual date and time of death is the time when medical personnel confirm that the brain is dead. Circulation is present, so it is important to know the exact moment when a person is considered dead.

2) confirmation of human death (in everyday practice) does not require so much accuracy. The circulation is stopped, no further life saving procedures are undertaken and then death is confirmed. The date and time is a certain indication of the completion of therapeutic procedures. We know, of course, that the brain dies later, but since we are not rescuing, the time difference between cardiac arrest and brain death can be missed.

I dont know about guidelines in other countries, but in Germany the "standstill" time is not routinely used for time of death. In instances like Jan Stachurski displayed it might be used - but that assumed that we are not even beginning resuscitation efforts because of patient wish, or other circumstances (such as when a person is found hours or even days late). So that is quite a different question!!

Let's actually assume we are actually initiating CPR on a person in cardiac arrest, like you are asking in your question:

Different factors in preclinical or clinical medicine are then used to evaluate potential outcomes for the patient, one of which is the (often estimated) length of time a person has not received chest compressions, EtCO2, ROSCs, ECG-rhythm etc., number of defibrillations, number of resuscitation cycles, manpower(how long can you actually hold through), time to next hospital, age, blood loss (e.g. traumatic resuscitation) [...] the list goes on and on.... all these variables themselves affect brain cell survival - meaning that when CPR efforts are interrupted, it usually means there is "nothing to save" - hence the interruption in the first place! (judicially, you need to be able to defend this decision, meaning that if there were credible signs of brain activity and the possibility of rescue, stopping CPR is not defensible). Time of interruption is then used as time of death, assuming the decision based upon the factors was correct - in my experience usually 40-50 minutes into resuscitation, but that again depends on the circumstances. This means that- in the form of a proxy- "loss of brain cells" is used to evaluate when someone can actually be pronounced dead.

Hope I could help :)

Death declaration needs 1 hour and 40 minutes after cardiac arrest in regular death and needs 14 hours after brain death...

After three minutes, global cerebral ischemia —the lack of blood flow to the entire brain—can lead to brain injury that gets progressively worse. By nine minutes, severe and permanent brain damage is likely. After 10 minutes, the chances of survival are low.