Time.deltaTime quite inaccurate

Questions & Answers
1

timer+=Time.deltaTime; I’ve always used everywhere. for simple timers of 1-60sec.
I’m aware of floating point issues with extensive use, but I recently found that doing this will get you an inaccuracy of .2 to .4 seconds for a 60 second period?!

I.e. Unity says 60 seconds have passed, in reality: 60.3 seconds have passed.
((version:2018.2.5f1))
Simple Update code to reproduce:

float t = 0;
double tt = 0; //more accurate bucket
System.Diagnostics.Stopwatch SW;
void Update() {
    if (t == 0) {
        QualitySettings.vSyncCount = 0;
        Application.targetFrameRate = 60; //20 =.2 error, 200 =~.4s error
        SW = new System.Diagnostics.Stopwatch();
        SW.Start();
        t = .00000001f; //negligible,  don't count deltatime this frame, cause that was previous time.
    } else {
        t += Time.deltaTime;
        tt += (double)Time.deltaTime;
    }
    if (t>60 && SW.IsRunning) {
        SW.Stop();
        Debug.Log("Float t="+t);
        Debug.Log("Double tt=" + tt);
        Debug.Log("StopWatch =" + SW.ElapsedMilliseconds/1000f);
    }
}

This seems like a huge error for just blaming floating point accuracy.

2

I am not sure exactly what causes the issue but I also get ± 0.2 seconds error with Update.

However if I use FixedUpdate the error is only about 0.003 seconds. So perhaps try that?

(Sorry I can’t really explain why this is the case though.)

3

Additionally, Just a sanity check against normal base-line float accuracy:

float myDeltaTime = 1f / 53f;
float myTimeSum = 0;
for (int i = 0; i < 53 * 60; ++i) myTimeSum += myDeltaTime;
Debug.Log("60 seconds of time slices:" + myTimeSum);

That Simple addition of 53fps worth of time slices for 60 seconds=
59.998
vs Roughly 59.7 from unity.

Unity MUST not be giving a complete and total accounting of time since previous frame? I wonder if editor time is being lost or some such. I guess that would be the next check, try a build to see if time still not adding up.

@rossadamsm: The fixed update has equivalent accuracy to the direct addition i’m doing, which makes sense, since it’s a simpler constant metric of 1/20f . Thanks for checking it :slight_smile: