Convert UIEvent timestamp to UNIX date - objective-c

Question

I use touchbegin function like
-(void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event;
{
touchStartTime = [event timestamp];
NSLog(#"Time of touch is %f ", touchStartTime);
}
but I would like to compare it with
double timeToCompare = [[NSDate date] timeIntervalSince1970];
but the format is not compatible (timestamp has some other reference point). How do I convert [event timestamp] to normal NSDate or vice versa?

Answer 1

As [event timestamp] is related to the system up time, you can get the [NSProcessInfo systemUptime] and get the current system time at the same time and go from there.
[[NSDate date] timeIntervalSince1970] - [NSProcessInfo systemUptime] + [event timestamp] == [compareDate timeIntervalSince1970]
(Well, I don't sign this in blood. Never tried it myself.)

Answer 2

Given that [UIEvent timestamp] is the number of seconds since the system was started, one approach to record the first event's timestamp and use that as a reference.
#interface MyClass ()
{
BOOL _haveFirstTimestamp;
NSTimeInterval _firstTimestamp;
NSTimeInterval _firstTimestampTime;
}
#end
...
-(void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event;
{
NSTimeInterval timestamp = [event timestamp];
if (!_haveFirstTimestamp) {
_haveFirstTimestamp = YES;
_firstTimestamp = timestamp;
_firstTimestampTime = [[NSDate date] timeIntervalSince1970];
}
NSTimestamp timeSince1970 = timestamp - _firstTimestamp + _firstTimestampTime;
NSLog(#"Time of touch is %f ", timeSince1970);
}