In Olson tzdata, many zones use LMT for the initial time zone offsets. LMT is calculated by the location and usually has zone offset with a fraction of minute. The minimum granular is seconds. Currently, GMT format pattern only includes hour and minute fields - GMT[+|-]HH[[:]mm]. With this representation, offset seconds will be lost. The GMT format pattern should support second field optionally. For example, GMT+02:13:42.
This enhancement was required for fixing #5990. All required changes were committed as a part of #5990.