periodic: Use locale-agnostic (ISO 8601) timestamp format

Instead of reyling on locale-sensitive output which can be mangled when e-mailed
use consistently ISO 8601 format which contains the same information as '-T'.

PR: 271240
MFC After: 3 days

(cherry picked from commit d2b4753f06dcabc090080b8c8c91bda00fc8dac3)