In his 1958 Grundlagen zur Entzifferung der Osterinselschrift Thomas Barthel identified two and a half lines of side a of Tablet C as containing a lunar calendar. There is not the slightest doubt that Barthel was right. In 1991 the Journal de la Société des Océanistes published an article in English by Jacques Guy analyzing those lines. Guy compared those lines with the names of nights collected by Thomson in situ in 1886, and with the phases of the moon during that period, as seen on Easter Island. He concluded that those lines were not a calendar strictly speaking, but an astronomical canon for telling in advance when to insert two intercalary nights, Hotu and Hiro, to ensure that the calendar anticipated the phases of the moon.

The image below shows the calendar, which starts on line 6 of side a of the tablet, approximately 160mm from its beginning. The hieroglyphs are colour-coded to show their probable function. In blue, the signs claimed by Guy to be taxograms for "night" (i.e. pure ideograms without a phonetic function), in green those claimed to be phonetic components, in black, red, and olive-green, distinctive groups of uncertain meaning.

Ca6
Ca7
Ca8
Ca9

According to Guy, the calendar is an astromical canon in two parts:

• A list of 28 "basic" nights, interspread with observational instructions (the groups in black and in red), and extending from Ca6:160 (first group in black) to Ca8:225 (last group in red). The groups at Ca7:115..135 and at Ca8:15..35 would be instructions to observe the apparent diametre of the moon and judge if it is small enough to warrant the insertion of an intercalary night (the moon when small is furthest from the earth, and therefore moves slowest). The groups in red with the fish coming up would merely mean "waxing" and those with the fish coming down "waning."
• A list of two intercalary nights, Hotu and Hiro (beginning of line 9, Ca9:0..10), introduced by a group of uncertain meaning (end of line 8, Ca8:230..250), perhaps phonetic approximations (the first sign looks like a turtle, honu for Hotu, the second, two figures sitting back to back, could be "two", he rua for Hiro)