Yes, you can automate this in Odoo using an Automated Action!

You want an automation that:

• Watches the Project Task model.
• When a task’s Planned Start Date is set or updated,
• It automatically sets the Deadline to 2 weeks (14 days) after the start date.
  
  

Here's how you can do it step-by-step:

• Go to:
  Settings > Technical > Automation > Automated Actions
• Create a New Automated Action:
  • Name: Set Task Deadlines Based on Project Planned Date
  • Model: Project (project.project)
  • Trigger: On Update
  • Trigger Fields: Planned Date (date_start)
  • Action To Do: Execute Python Code

if record.date_start:

    for task in record.task_ids:

        task.date_deadline = record.date_start + timedelta(days=14)

Important:

• This updates all tasks under the project when you change the project's start date.
• It will overwrite existing deadlines of tasks — unless you add conditions to skip tasks with already set deadlines (optional — I can show you that if needed).
  
  I hope it will help you. If you need more help, feel free to contact meme at bitlevelcode@gmail.com

Here is a screenshot of the automated action: