Performance data is empty when giving startTime and endTime

Hi I tried to get host performance data. I tried the following code:

counterInfo = {}
        for c in perfManager.perfCounter:
            #print(c)
            #time.sleep(500)
            prefix = c.groupInfo.key
            fullName = c.groupInfo.key + "."+c.nameInfo.key + "." + c.rollupType
            #print(c.groupInfo.key, ' ',c.nameInfo.key,' ',c.rollupType, ' ',c.unitInfo.key)
            counterInfo[fullName] = c.key
for child in content.rootFolder.childEntity:
            datacenter = child
            hostFolder = datacenter.hostFolder
            hostList = hostFolder.childEntity
            for hosts in hostList:
                esxhosts = hosts.host
                for esx in esxhosts:
                    summary = esx.summary
                    esxname = summary.config.name
                    counterIDs = [m.counterId for m in perfManager.QueryAvailablePerfMetric(entity=esx)]
                    metricIDs = [vim.PerformanceManager.MetricId(counterId=c, instance="*") for c in counterIDs]
                    spec = vim.PerformanceManager.QuerySpec(maxSample=1, entity=esx,intervalId=20,startTime=startTime, endTime=endTime, metricId=metricIDs)
                    result = perfManager.QueryStats(querySpec=[spec])
                    print(result)

But I got empty result.
Again If I remove startTime and endTime from the QuerySpec then I am getting some value. Can someone please explain why this is happening?

Start with the QuerySpec and QueryStats usage shown in the issue, then review how pyVmomi represents startTime, endTime, intervalId, and available performance metrics. Reproduce the query with and without the time bounds and inspect the returned result. Done means the reason for the empty time-bounded result is documented and the issue identifies the required query conditions or correction.