Full Answer

The window you choose in your ad platform is an instruction, not a guarantee. It only works if the identifier set at click time still exists when the purchase happens. On Safari, that's where it breaks. Intelligent Tracking Prevention expires script-set cookies after seven days, and trims that to 24 hours when the visitor arrives from a referrer ITP treats as a tracker, which covers most paid traffic. Set a 30-day window all you like; for Safari users the cookie is gone first.

This is why stores see paid conversions quietly undercounted on Apple devices while the same campaigns look fine on Chrome. The customer who clicked an ad, thought it over for ten days, and came back to buy is invisible to last-click attribution because the click identifier didn't survive. The window didn't fail; the storage did.

Restoring a real window means not relying on the browser to remember the click. Server-side tracking sets a first-party identifier from your own server and reconciles the conversion against first-party data such as a hashed email or order record, so a 30-day or 90-day window holds even for Safari. The practical test is simple: if your reported window depends on a JavaScript cookie, Safari has already shortened it for you.