Works copyrighted before 1964 had to have the copyright renewed sometime in the 28th year. If the copyright was not renewed the work is in the public domain. It is best to search 6 months before and after the required year. Some magazines are published the month before the cover date and some registrations may be delayed for a few months.
This December 23, 1929 issue of Time would have to be renewed in 1956. Online page scans of the Catalog of Copyright Entries, published by the US Copyright Office can be found here: *http://onlinebooks.library.upenn.edu/cce/
The search of the Renewals for Periodicals for 1955, 1956 and 1957 show no renewal entries for Time. The publishers, Time Inc., started renewing the copyrights of Time magazine in 1964 with the July 6, 1936 issue. Most (if not all) issues that were published before July 1936 are in the public domain.