Methods to determine the delay between a real world event and timestamping in ROS?

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Hello,

So I want to determine the time it takes for a sensor to timestamp an event that takes place in the "real world". The way my sensor works, is that the sensor sees the event happening, it checks if this detection is valid and then it simply puts a timestamp on this detection in ROS. I want to know how long it takes between it detecting the event and eventually timestamping this, which in other words is the response time from the sensor to ROS. I have searched for this for a long time but I simply can not find anything about this.

Any answers,hints or advice is appreciated!