Use a dated travel chronology to evidence the entry date.
Arrange every document that places the border crossing in time. This can include transport records, reservations, messages, payment records and documents showing a stay outside the Schengen area. The documents should form a coherent account of the same journey.
Under Article 12 of the Schengen Borders Code, a missing stamp triggers a legal presumption. Any credible evidence can rebut that presumption. One document does not necessarily have to explain the entire journey.