Wrong calculation of Month Days for restrictions using validity periods

Current Situation

• If a Month Days restriction is used that holds a validity period, then target dates of a schedule are wrongly calculated:
  • For the first month the begin of validity is added to the month days.
  • Instead, the begin of month should be used when calculating month days.
• This applies to the schedule's preview and to orders created by the Daily Plan.

How to reproduce

• Assume a working day calender holding all days of year using the frequency "Every day"
  • Specify the calendar validity: 12.12.2024 to 31.12.2025, see attached c1.workingdayscalendar.json
• Add a schedule based on the above calendar, see s1.schedule.json
  • Add a restriction to the schedule using the frequency: Month Days (Count from First Day)
  • Specify month days: 18th and 19th of month, see s1.schedule.json
  • Specify validity: 13.12.2024 to 19.02.2025
• Verify wrong calculation:
  • The 29th and 30th of December 2024 are incorrectly displayed instead of 18th and 19th, see show_preview.png. The reason being that for the first month the begin of validity, 13.12.2024, is added to the month days 18th and 19th which results in 29th and 30th of December.
  • The Daily Plan is similarly created for 29th and 30th December 2024.

Desired Behaviour

• The Month Days frequency should calculate days of month from the begin of month instead of the begin of validity.