use get datetime interval, not make it

[peterdudfield]

Pull Request

Description

Error caused by datetimer interval already existing

Fixes https://github.com/openclimatefix/uk-analysis-dashboard/issues/15

How Has This Been Tested?

CI tests

• [ ] Yes

Checklist:

• [ ] My code follows OCF's coding style guidelines
• [ ] I have performed a self-review of my own code
• [ ] I have made corresponding changes to the documentation
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] I have checked my code and corrected any misspellings

[codecov[bot]]

Codecov Report

Merging #25 (593060a) into main (cfd6746) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main      #25   +/-   ##
=======================================
  Coverage   99.43%   99.44%           
=======================================
  Files           8        8           
  Lines         357      358    +1     
=======================================
+ Hits          355      356    +1     
  Misses          2        2           

Impacted Files	Coverage Δ
nowcasting_metrics/app.py	95.83% <100.00%> (+0.08%)	:arrow_up:

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