It is well known that the first sunspot groups of a new cycle appear before sunspot minimum and continue to emerge for 12 or more years, ending after the following minimum. The period of time over which activity belonging to a specific cycle is observed is called the extended activity cycle. A criterion for differentiating sunspot groups into their proper solar cycles during the overlapping, transitional period is proposed. The monthly sunspot group numbers during each extended activity cycle are constructed to show characteristics of solar activity for them. On the basis of the defining characteristics we estimate the conventional start of the beginning time of sunspot cycle 24 (the next cycle) to occur ~2007.2 ¿ 1 year, at the 68% percent level of confidence.