Timing an LDR in C++

[Ragingenferno]

I'm so confused on this topic. I'm doing this for a arduino mega (pretty sure arduino is C++) What I'm trying to do is this: Read the value back from the LDR (Light Dependent Resistor) and start a timer for when the value is in a certain range and perform a function if the time recorded is a certain amount of time.

For example: I will have the LDR taped to a screen. SO when the screen is black (LDR value is about 120-150), it starts timing how long it was black for and keeps timing until the screen displays light (LDR 600 - 620). Hypothetically, lets say the screen was black for about 12000-12600ms, if the time recored falls between that, it does something but if its above 12600ms it does something else. (If, else statement)


However, all I know how to do is get the value of the LDR, I don't know anything else.

Can someone help me with the code for the timing function, I'm lost.

If someone can pleaseeeeeeee help that would be the best thing ever