Frequency : TIME

How do they communicate on the same day through time trvale when time and date is man made. hmmm

Date is man made, but the Earth going round the Sun is natural enough to count.
Plus they will never gonna explain how the time connection is possible, but if there ever has to be an explanation it would be something like Intestellar, so entities able to relate with the understanding of men.

In the movie version it was because of the Aurora Bourialis bouncing of sattilites and the signal was only workable when the Aurora was above the antenna at the cops house. In this version I imagine they will come with an explanation but it's only been two episodes so far.