Use Arrow's shift() instead of replace() to modify dates

[vincentbernat]

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Previously, the replace() method from arrow was shifting the date when the arguments were using the plural form. Since Arrow 0.9.0, this has been deprecated in favor of a shift() method. Arrow 0.14.5 completely removed the ability for replace() to shift dates. This leads to errors like AttributeError: unknown attribute: "hours" when using plural form.

This commit replace the use of replace() by shift() since the intent is always to shift the current date.