What do the "days of service" on Tracfone minutes cards mean?

I recently bought a Tracfone for a friend who is in the hospital. I've never used a pay-as-you-go phone myself; I've always been with Verizon standard plans. What I don't understand about the minutes cards is that they say "400 minutes and 365 days of service" or "30 minutes and 30 days of service," etc. What do the "days of service" mean? Does that mean that the minutes expires in those amount of days? I know I've heard that most pay-as-you-go phones have the minutes expire if they aren't used or renewed in 15 to 30 days. Could someone please help? I only got this for him because I found out the pay-as-you-go phone that he did have was some oddball company meant for calling Latin America and the minutes could only be bought at random sketchy bodegas in the bad part of town, not in all major retailers, supermarkets and pharmacies like all the other major phones (T-Mobile, Virgin, etc.). Any help understanding this would be appreciated.