The PTB (Physikalisch-Technische Bundesanstalt) is Germany’s national metrology institute, based in Braunschweig. It operates the atomic clocks that define Germany’s legal time and provides public NTP time servers (ptbtime1.ptb.de, ptbtime2.ptb.de). For your ISMS, the PTB is relevant because ISO 27001 Annex A.8.17 requires clock synchronization against a trusted reference. In Germany, the PTB time servers are the natural choice. Correct time synchronization is a prerequisite for reliable log analysis, forensic investigations, and meeting reporting deadlines for security incidents.