2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/35320We present some basic facts concerning simultaneity in both special and general relativity. We discuss Weyl's proof of the consistence of Einstein's synchronization convention and consider the general relativistic problem of assigning a time function to a congruence of timelike curves.Latex2e, 9 pages, no figures. To appear in the proceedings of the school 'Relativistic Coordinates, Reference and Positioning Systems', Salamanca (Spain), January 21-25, 2005General Relativity and Quantum Cosmologytext