This is completely wrong and it doesn't even make any logical sense..."sending it in the past". What does that even mean? Any date you send the letter will be "the past" relative to when it reaches the recipient; there's nothing wrong with backdating an application. Do you even proofread what you write?
Anyways, straight from the instruction manual:
1) Note: Your application will be returned to you if it is:
- not signed and dated
- dated more than 90 days before we receive it
- post-dated (dated into the future).
So it's okay to keep the old dates so long as they are within 90 days of which your updated application will reach the CIC
2) You actually have THREE dates the change. The date you input into the calculator, the date you sign with in the calculator printout, and the date in the application, not one. And all three dates should match. Oh, and you have to change the dates in the application as well (I believe on page 2).
This is such a hassle rife with mistakes that it's better to just leave the old dates as is and hurry to send them back. The amount of bad advice people give here is ridiculous.