Hyatt Gift Checks and "Stay Certificates" were sold by Hyatt to consumers and businesses as travel incentive instruments until a few years ago when sales were discontinued. However, most were promoted by Hyatt and specifically state on the certificate that they will "never expire".
So far, Hyatt is honoring the non-expiration side of the bargain (as they would legally be responsible to do) but although they are now seen less often by hotels they are sometimes refused at the front desk. Also, some hotels never participated in the first place. Here's a link from the current Hyatt website that shows which properties accept which product whether it be gift certificates, gift cards or stay certificates. Just scroll down from the gift card pull down menu:
Participating Locations - Hyatt Incentive Rewards
As I still possess well over 100 stay certificates and gift certificates, I'm pretty familiar with them. If a property is unaware about how to process them or other information, instruct them to go to the Hyatt Knowledge Base for complete information.