When the system switch to slow mode from normal mode, (refer to system operation mode section in datasheet), the system internal low-speed oscillator is running, the oscillation rate can refer to datasheet system low clock section, typically, when 5V,32KHz, when 3V,16KHz. 

If the timer is used at this time, the system clock(Fosc) is no longer an external oscillator (external crystal / ceramic oscillator / RC oscillator), but an internal low-speed oscillator, timer calculation method is the same with external oscillator. Such as: under the conditions of work in 5V,the system Flosc will be 32KHz, as MCU hardware settings Fcpu = Flosc/4 when enter low-speed mode, if the timer frequency divider sets to 1, then each time count becomes 1/(32KHz/4) = 0.125ms. Note: In RTC mode, when enter low-speed mode, T0 clock source remains external 32.768K.