This object indicates the last time - in milliseconds - at which the peak thread count was reset to the current thread count. This corresponds to a time stamp as returned by java.lang.System.currentTimeMillis(). Setting this object to a time earlier than its current time value has no effect. Setting this object to a time later than its current time value causes the peak thread count statistic to be reset to the current thread count. The new value of this object will be the time at which the reset operation is triggered. There could be a delay between the time at which the reset operation is triggered and the time at which the actual resetting happens, so this value is only indicative. See java.lang.management.ThreadMXBean.resetPeakThreadCount()