We present an efficient algorithm to optimize routing and grooming in IP-over-WDM networks where traffic is generated by backbone nodes located in different time zones in a global network. We analyze the performance of our algorithm, called RUTZ (Routing Using Time Zones), according to the cost and link utilization as well as observed significant improvements compared to traditional grooming approaches. The cost gain and link utilization in RUTZ gets closer to the dynamic on-demand cost assignment method which adjusts the provisioned bandwidth according to user demands on an hourly basis. In a global network, the daily traffic profile in a backbone link shifts as the time changes, and the peak and valley move as the sun moves. If time-zone dynamism is predictable, traffic can be groomed and routes can be calculated in an efficient manner without making periodic (e.g., hourly) changes in bandwidth allocation. In this study we present an intelligent routing and grooming algorithm which exploits this dimension of dynamism in IP traffic.