GPS is beginning to be more and more cost effective! There are now smaller and smaller ready made modules that may make sense for various projects where a more "world view" sense of time accuracy is important to a product.

The long wave RF time reference scheme referenced by Charles is probably the lowest cost scheme to get accurate time reference information. It does have a certain "region centric" nature to it as Charles and Steve have pointed out but for localized product deployment it can be a very effective scheme.

Michael Karas