This paper presents mechanisms for describing real-time tasks in functional reactive programming (FRP) languages for small-scale embedded systems. We have designed and implemented Emfrp, an FRP language for resource-constrained systems, and demonstrated its usefulness with several applications. However, the language requires using external clocks as time-varying values when describing time-dependent behaviors. In this work, we extend the types of time-varying values that express their update timings to describe periodic and aperiodic tasks. The extensions enable concise and precise descriptions of various timed behaviors. We evaluate prototype implementations of the extended languages concerning program size, execution time, and power consumption.