React Native get relative time

TimerModule.h

#import <Foundation/Foundation.h>
#import "RCTBridgeModule.h"

@interface TimerModule : NSObject <RCTBridgeModule>

@end

TimerModule.java

import com.facebook.react.bridge.Promise;
import com.facebook.react.bridge.ReactApplicationContext;
import com.facebook.react.bridge.ReactContextBaseJavaModule;
import com.facebook.react.bridge.ReactMethod;

public class TimerModule extends ReactContextBaseJavaModule {
    public TimerModule(ReactApplicationContext reactContext) {
        super(reactContext);
    }

    @Override
    public String getName() {
        return "Timer";
    }

    /**
     * Return a relative unit of time in seconds that cannot be influenced by the user
     * @param promise
     */
    @ReactMethod
    public void getRelativeTime(Promise promise) {
        // System time in milliseconds
        long time = android.os.SystemClock.elapsedRealtime();

        // React Native bridge complains if we try to pass back a long directly
        promise.resolve(Long.toString(time / 1000));
    }
}

TimerModule.m

#import "TimerModule.h"
#include <sys/sysctl.h>

@implementation TimerModule

RCT_EXPORT_MODULE(Timer);

/**
 * Return a relative time in seconds that can't be tampered with by the user
 */
RCT_EXPORT_METHOD(getRelativeTime: (RCTPromiseResolveBlock)resolve rejecter:(RCTPromiseRejectBlock)reject)
{
    resolve([NSString stringWithFormat:@"%ld", [self uptime]]);
}

/**
 * Get the system up time in seconds
 *
 * Method originally from here:
 * http://stackoverflow.com/questions/12488481/getting-ios-system-uptime-that-doesnt-pause-when-asleep
 */
- (time_t)uptime
{
    struct timeval boottime;
    int mib[2] = {CTL_KERN, KERN_BOOTTIME};
    size_t size = sizeof(boottime);
    time_t now;
    time_t uptime = -1;

    (void) time(&now);

    if (sysctl(mib, 2, &boottime, &size, NULL, 0) != -1 && boottime.tv_sec != 0) {
        uptime = now - (boottime.tv_sec);
    }

    return uptime;
}

@end