该内容出自德国工程师写的教科书。
When a timer is started, the CUP uses the programmed time value as the timer's running time. The operating system updates timers at fixed intervals and independently of the user program scan, that is ,it decrements an active timer's time value as per the time period indicated by time base.

When the value reaches zero, the timer is regraded as expired. The CPU then sets the timer status(signal state"0" or "1",depending on the type of timer involved)and drops all futher activities until the timer is started again.

when a time value of zero is specified, the timer remains active until the CPU processes it and discovers that the timer has expired.

Timers are update asynchronously to the program scan. It is therefore possible that the status of the timer at the start of the cycle may differ from its status at the end of the cycle. If you use timer instructions at only one location in the program and in the suggested order(see below), there will be no errors due to asynchronous timer updating.
从这段话中我们能看到定时器是异步于程序扫描周期的，也就是说在一个程序的扫描周期内，定时器的状态可能出现两种形式，正如上面所言，可能在程序的头，定时器状态为0，可能在程序的尾部，定时器状态为1。