darling-cocotron/QuartzCore/CABase.m

#import <Foundation/NSString.h>
#import <QuartzCore/CABase.h>
#import <mach/mach_time.h>

static double conversionFactor() {
    struct mach_timebase_info timebase;
    kern_return_t error;

    if ((mach_timebase_info(&timebase)) != KERN_SUCCESS) {
        NSLog(@"mach_timebase_info returned %d", error);
        return 1;
    }

    return 0.000000001 * ((double) timebase.numer / (double) timebase.denom);
}

CFTimeInterval CACurrentMediaTime(void) {
    uint64_t value = mach_absolute_time();

    return ((double) value) * conversionFactor();
}