47 CFR § 73.1705 - Time of operation.
(a) Commercial and noncommercial educational TV and commercial FM stations will be licensed for unlimited time operation. Application may be made for voluntary share-time operation.
(b) Noncommercial educational FM stations will be licensed for unlimited and share time operation according to the provisions of § 73.561.
(c) AM stations in the 535–1705 kHz band will be licensed for unlimited time. In the 535–1605 kHz band, stations that apply for share time and specified hours operations may also be licensed. AM stations licensed to operate daytime-only and limited-time may continue to do so; however, no new such stations will be authorized, except for fulltime stations that reduce operating hours to daytime-only for interference reduction purposes.