How to use the variable declared in one class in another class in Obj C

I have a NSDate *date1 in class1 implementation file(I initialized it as Global variable). I have NSDate *date2 in class 2 implement开发者_StackOverflowation file (initialized it as Global variable). I need to calculate the NSTimeInterval between the two dates in class 2. But I could not do it. I could not access date1 in this class. It is giving error as (date2 undeclared). Please give me help in how to call other class variables in this class.

Thank you.

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You need to refer to date1 with extern:

// class1 file:
NSDate* date1 = nil;

// class2 file:
extern NSDate* date1;
static NSDate* date2 = nil;  // remove the static if this is needed in other files.