Protocol specification - correct name for the timestamp/event-counter column

Question

I am developing a communication protocol between a Computer and a Device, using a USB connection. The device has multiple sensors, each one measuring different types of variables in different sample rates.

Each time that the measurement from a sensor is ready the Device sends a message to the Computer as follow:

Sensor ID	Event Counter	...	Payload
0x01	0x00000012	...	123.123f

This means that the sensor 0x01 will send the measurement of the event 18 (decimal). The next event will be:

Sensor ID	Event Counter	...	Payload
0x01	0x00000013	...	012.120f

Now, in a future release of this protocol the user will have the option to select between an Event Counter or a Timestamp. So, for example instead of the format above with the Event Counter, it will be possible to have something like:

Sensor ID	Timestamp	...	Payload
0x01	0x00004120	...	012.120f

This means, that since the device started streaming, it already elapsed 16672 milliseconds (decimal) until that event.

So, for the second column it will be an Event Counter or a Timestamp, but for the documentation, I want to have a unique name to represent that column, which must encapsulate both the Event Counter and Timestamp idea. What is the correct name for that protocol column?

Sensor ID	(INSERT THE CORRECT NAME HERE)	...	Payload
0x0X	XxXXXXXXXX	...	XXXXXXf

Thanks a lot!