Just reading KLM travel advice, it states COVID negative test is valid for up to 96 Hours.
If flight delay is proven to be beyond doubt, no fault of the passenger.
,
TLDR : Answer Summary
KLM states that COVID negative tests are valid for up to 96 hours, but it's important to note that the airline does not set the entry rules for the destination country. If a flight delay occurs, and it is proven to be beyond the passenger's control, the implications on their ability to enter the country due to the test's expiration remain unclear.