T-Mobile's Law Enforcement Relations Group provides information on the timestamp format used in call detail records. Current records are in UTC, while older records are in the user's local time zone. The notice includes a table of record types and their associated time formats.
documents/vol00010-efta01342687-pdf.pdf
{"document_type":"correspondence","key_locations":["Parsippany, NJ"],"document_subtype":"notice","key_organizations":["T-Mobile US, Inc.","Metro PCS"],"reference_numbers":["SDNY_GM_00170623","EFTA01342687"]}