BUG: [GH 11738] Fix for end_time when multiple of a frequency is requested
closes #11738 
1. Added a new test with the multiple of a frequency.
2. Rearranged some tests to improve readability of the code.
3. Added bug fix to whatsnew
4. Added a comment in the test

Didn't follow the contributing guidelines carefully in PR #11771 and hence this new PR. 
