I worked on a project like this sometime back, We used WWVB at 60 Khz because
the VLF frequency can be received more reliably at different times of the day.
The receiver we used was somewhat complicated (14 transistors) because this
project was done about 10 years ago, since then NIST has tripled the power
output of WWVB thats why suddenly the market was full of low cost WWVB
radio clocks. To decode the data stream you have to take into account noisy signals that may occur at times. Most of these clocks including the one I
worked on do not continusly receive the radio signal they have there own RTC
IC to keep the time and sync it to WWVB one or more times a day.