Change timestamp UG, format

[koh-jx]

No description provided.

[jeffsieu]

I believe the GUI also needs to be changed to display in dd-mm-yyyy. But that could be done in a different PR.

[koh-jx]

I believe the GUI also needs to be changed to display in dd-mm-yyyy. But that could be done in a different PR.

This is done in the Timestamp#toString()

[codecov-commenter]

Codecov Report

Merging #140 (4de78b2) into master (7f739b6) will increase coverage by 0.36%.
The diff coverage is 66.66%.

@@             Coverage Diff              @@
##             master     #140      +/-   ##
============================================
+ Coverage     58.41%   58.78%   +0.36%     
- Complexity      599      602       +3     
============================================
  Files           121      121              
  Lines          2561     2562       +1     
  Branches        251      251              
============================================
+ Hits           1496     1506      +10     
+ Misses          975      964      -11     
- Partials         90       92       +2     

Impacted Files	Coverage Δ
.../main/java/seedu/address/model/task/Timestamp.java	61.11% <66.66%> (+14.05%)	⬆️
...in/java/seedu/address/commons/util/StringUtil.java	89.28% <0.00%> (+25.00%)	⬆️

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[jeffsieu]

@koh-jx Conflicts need to be resolved.