How can I fix my iPhone's incorrect purchase date?

You can contact Apple support to get the date corrected. But as mentioned, your warranty has likely expired anyway, unless you bought Apple Care+ or are in Europe. (EU requires 2 years of warranty).

You can try to get it corrected by contacting Apple Support though.

click here ➜ Update your proof of purchase - Apple Support

Whether it's expired or not, The April 1st 1978 date is still an error. So it should be corrected regardless.

There is nothing to fix.

The purchase date of 1 April 1978 indicates that you bought a used or refurbished iPhone.

But, even if you bought a new phone about 2 years ago, the standard AppleCare warranty would have expired a year after the purchase date.

At this point, you will have to pay for any service or repairs that might be required on your phone.