Timer in Arduino

I want to imploment a timer to my Code so i know how long it has benn running so i can like say:
t+0.1s
t+0.2s

and Afterwards Create a Graph wit the Coresponding Sensor values.
But i dont know how to do it!!
HELP

millis() function tells you how long the Arduino has been running in milliseconds. It rolls over after 49 days.

Depending of the arduino board type you are using, you can configure its timer to interrupt on the time interval you want, and set the interrupt to do whatever you intend to do.
You can use overflow, ctc mode, etc..
Rgrds

Yeah and what when i would like to start the Timer at a certain Point ?

1 - Record millis() at the start time.
2 - Record millis() at the end time.
3 - Subtract start time from end time.
4 - Save difference in a array to be used in your graph.

erow2019:
Yeah and what when i would like to start the Timer at a certain Point ?

You mean you want to have a sort of schedule?
If it is the case, need to add a real time clock to your system.

erow2019:
Yeah and what when i would like to start the Timer at a certain Point ?

It depends on what you mean by "certain point". If you mean at an interval that does not have to coincide with a particular time of day (TOD) then you can just use the millis() time. If it needs to correspond to a specific TOD then you will need a real time clock (RTC) that can provide TOD.

erow2019:
Yeah and what when i would like to start the Timer at a certain Point ?

unsigned long certainPoint;
...
certainPoint = millis(); // start the timer
...
Serial.print(millis() - certainPoint);