Nowadays, the Internet is part of our lives, as an essential way of communication and information. However, it suffers from limitations that prevent guarantees of Quality of Service (QoS) to the users. Thus, Internet Service Providers (ISPs) need to evolve, adding new technologies and management strategies, such as Network Virtualization (NV), Software Defined Network (SDN) and Network Function Virtualization (NFV), resulting in the Future Internet Service Providers (FISPs). FISPs slice their network resources among clients and delivered services, where an algorithm to define the structure of each slice is crucial to support Quality of Service (QoS) and resource utilization. Therefore, FISPs should consider the elastic resource utilization of the clients within periods of time through the day. In this context, this paper presents the Per Time Calculation (PETIC) algorithm to define slices based on the daytime bandwidth requirements of the clients. Numerical results suggest that the proposed algorithm defines more suitable slices than existing approaches.