Fix date idate subtraction

[ben-schwen]

Closes #7825
Follows #7827

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.01%. Comparing base (81876f7) to head (27bfb97).
⚠️ Report is 7 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7828   +/-   ##
=======================================
  Coverage   99.01%   99.01%           
=======================================
  Files          88       88           
  Lines       17228    17240   +12     
=======================================
+ Hits        17059    17071   +12     
  Misses        169      169           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[MichaelChirico]

Looks good at a glance. Let's merge the atime test first so we can see the performance change directly in the PR.

[github-actions]

• HEAD=fix_date_idate_sub faster P<0.001 for DT[by,verbose=TRUE] improved in #6296
  

Generated via commit 708974b

Download link for the artifact containing the test results: ↓ atime-results.zip

Task	Duration
R setup and installing dependencies	6 minutes and 20 seconds
Installing different package versions	12 minutes and 45 seconds
Running and plotting the test cases	10 minutes and 15 seconds