fix duration computation

[ritchie46]

fixes #3786

[codecov-commenter]

Codecov Report

Merging #3790 (9404e24) into master (bdb1dd0) will decrease coverage by 0.08%.
The diff coverage is 91.35%.

@@            Coverage Diff             @@
##           master    #3790      +/-   ##
==========================================
- Coverage   63.01%   62.92%   -0.09%     
==========================================
  Files         439      440       +1     
  Lines       71853    72452     +599     
==========================================
+ Hits        45280    45594     +314     
- Misses      26573    26858     +285     

Impacted Files	Coverage Δ
polars/polars-lazy/src/dummies.rs	0.00% <0.00%> (ø)
polars/polars-lazy/src/logical_plan/aexpr.rs	79.51% <ø> (ø)
polars/polars-lazy/src/logical_plan/alp.rs	77.14% <ø> (-0.20%)	⬇️
polars/polars-lazy/src/logical_plan/mod.rs	90.90% <ø> (ø)
...lars/polars-lazy/src/logical_plan/optimizer/mod.rs	100.00% <ø> (ø)
...c/logical_plan/optimizer/predicate_pushdown/mod.rs	79.13% <ø> (ø)
...src/physical_plan/executors/groupby_partitioned.rs	80.92% <ø> (-0.10%)	⬇️
...rs/polars-lazy/src/physical_plan/executors/join.rs	74.46% <0.00%> (-0.81%)	⬇️
...ars/polars-lazy/src/physical_plan/executors/mod.rs	90.84% <ø> (ø)
...polars-lazy/src/physical_plan/expressions/apply.rs	93.41% <ø> (ø)
... and 91 more

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