fix: handle negative due date for service levels

[ccailly]

Changes description

Checklist

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• [ ] Tests for the changes have been added
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Closes #N/A

The plugin allows you to use the response of a date-type question to define a service level. In the properties configuration of a form destination, you can define the TTR from a specific question and deduct/add days. When the number of days/hours/minutes... was negative, the value was always interpreted as a positive value.