Handling dates

I have questions about the steps to manipulate dates.

Are the steps below ok? Can I start my studies?

• I have the Date attribute and want to insert 2 days.
• Convert Date to String .
• Convert String to Calendar .
• I make the code to manipulate the date obtained.
• Converting from Calendar to String .
• Converting from String to Date .
• Saved.

Why should I try to declare as Calendar when saved from this error?

  Unresolved compilation problem: Type mismatch: can not convert from

Code for more details:

// data em que o dinheiro foi entregue
    @DateTimeFormat(pattern = "dd/MM/yyyy")
    @Temporal(TemporalType.DATE)
    private Date dataEmprestimo;

If I put private Calendar dataEmprestimo , it gives the error mentioned above.