Arduino with magnetic sensor...Help please.!

I use an arduino uno with a magnetic sensor because i want to create a parking spaces availability.! I have already finish the code to detect the change of the sensor values when a car park over the sensor.. And now i want to write a code to find the time of this change.. What is the easiest method.?? I am thinking to send the data into an array and compare the values so i can find the change between the values.. I am not sure how can i do it.. If anyone can help me with his advice or with example of code please reply..!! Thank you.!

In this picture are the values that i take from the sensor.. the grad value change when i have magnetic field over the sensor.. As you can see grad=149.08, grad= 147.65, grad= 148.53, grad= 149.08 and after that i place magnetic field go to grad= -15.11, grad= -130.60, grad= -129.89, grad= -130.60, grad= -129.89. This is the change that i want to detect and for example after that i will turn on the led.!

magnetic.jpg309×734 84.6 KB

Zinonas:
now i want to write a code to find the time of this change.. What is the easiest method.??

There is a Time library that can help: Arduino Playground - Time

You will probably want to add a Real Time Clock since the Arduino is not great at keeping time by itself.

http://playground.arduino.cc/Main/DS1302
http://www.ladyada.net/learn/breakoutplus/ds1307rtc.html

You can use millis() to determine, relative to when the Arduino started, when the value changed to "car parked here" or when it changed to "no car parked here". The difference is how long the car was parked there, in milliseconds. Converting that to seconds, minutes, hours, days, etc. is simple math.

Thank you very much for help..!! It was very important for me.! Thank you again.!