Cheapest way to synchronise date and time for standalone system?
I would use WWVB (actually DCF77, since I'm in Europe). You can buy compact low power receivers for 10 to 15 dollar. This one is GBP 7.95 (USD 12.50).



It works at 3 V to 5 V, and has an enable input to switch down from a few mA to less than a \$\mu\$A. Thanks to its small size it can easily be built in into your product.

GPS may not have reception inside.