Improve util::meta::time_to_string

[ken-matsui]

No description provided.

[codecov]

Codecov Report

Merging #368 (dcecdfe) into master (53ce5c8) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #368   +/-   ##
=======================================
  Coverage   78.15%   78.15%           
=======================================
  Files          52       52           
  Lines        2417     2417           
=======================================
  Hits         1889     1889           
  Misses        528      528           

Impacted Files	Coverage Δ
include/poac/util/meta.hpp	46.51% <ø> (ø)

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