When Jill asks Chuck when they should meet at Buy More he gives her a "geek code" so Casey won't be able to follow them. Chuck tells Jill to take the latent heat of aquatic fusion (in calories/gram) and then to subtract the atomic number of the element thulium. The latent heat of aquatic fusion (the heat required to turn ice to water) is approximately 80 calories/gram. The atomic number of thulium is 81. 80 - 81 = 1. Chuck and Jill went on their date while the sun was up so Chuck probably intended for Jill to meet him at 1 o'clock in the afternoon, right after lunch!