Enhance Time class static method and its variables

[peter279k]

How to report a issue on GitHub

• The setDateFormat method is static, using the self to do static method call, not $this.
• Let $dateFormat be static variable because the static method only allows the static variables.

[codecov[bot]]

Codecov Report

Merging #257 into master will not change coverage. The diff coverage is 0%.

@@           Coverage Diff           @@
##             master   #257   +/-   ##
=======================================
  Coverage       8.4%   8.4%           
  Complexity     1889   1889           
=======================================
  Files           110    110           
  Lines          4782   4782           
=======================================
  Hits            402    402           
  Misses         4380   4380

Impacted Files	Coverage Δ	Complexity Δ
src/Time/Time.php	79.24% <0%> (ø)	24 <0> (ø)	:arrow_down:

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