When Chats Split Apart: Formalizing Temporal Communication Fragmentation and the CARTA Reconstruction Framework

When a phone fails and its owner keeps the messaging account running on a second device, the account's history can end up in two archives, each intact and each missing what the other holds. We call this Temporal Communication Fragmentation (TCF), a loss by non-union: no message is deleted, but the platform offers no operation that joins the two halves. WhatsApp's own documentation states that old and new chats cannot be merged. We define TCF by three jointly necessary and sufficient conditions and specify CARTA 1.1, a framework for reconstructing one archive from several. CARTA is an output contract owed to a reader who cannot check the result. Records that cannot be read unambiguously are quarantined with a typed reason, and a report gives each record's provenance, the conflicts and deduplication ambiguities found, and every setting used. We evaluated a reference implementation, CHAMP-Pipeline, on synthetic fragment sets with known ground truth. With its declared date inference it reproduced the ground truth in all 4,500 configurations of a parameter sweep; the invariants and the predicted outcome held in all 5,000 random configurations of two to four fragments; and reconstruction stayed exact at one million messages. A zone-offset advisory, frozen before testing, flagged all 2,800 misdeclared zones in a designed test of overlapping fragment pairs and 1,470 of 1,542 in random configurations, and raised false alarms in 2 of 600 correctly declared runs. A systematic search found 60 chat-merging tools; none fully provides more than 2 of the five output properties CARTA specifies. With a fragment's time zone left undeclared, every benchmarked tool and CHAMP-Pipeline produce the same duplicated and misdated records, and only CHAMP-Pipeline's report says so. All data are synthetic; no real archive has been reconstructed.