I may believe it once the media have finished carbon-dating the ink in the diary to verify that it was written on the day he met Hillary. It’s very possible he wrote it the next day. And carbon-dating isn’t accurate down to the day so we can already assume he’s wrong or that the tests will be inconclusive. /s
And, of course, to be absolutely thorough, the 'authorities' would need to test every sheet and binding to ensure that there were no modifications. When the 'inconclusive' results are announced and Mr Woods asks for his diary back, the 'authorities' will assume a shocked expression and say that such through testing required the destruction of the book. In the background, Hillary just bobbles and smiles!