You can use a 433 Mhz, or the other frequency 315? Mhz, radio frequency pair. With the VirtualWire library. As long as only air is in between them. As they are line of sight radios. They are also cheep. They only work one way. If you want 2 way you need a 433 pair going one direction, and the 315? Pair going the other way. They only require, positive voltage, negative, and a signal wire. Out of the box the range is really low. But adding an antenna, piece of wire, drastically increases the range. As for the time of day thing, you might want to add a RTC module (Real Time Clock). The arduino crystal isn't good at keeping time, it over a days worth of time it can be off by a few minutes. Multiplied over weeks and you are hours off.