(CNN) -- For a man used to tracking his TV ratings -- as well as his poll numbers -- the measurement of President Donald Trump's first 100 days is a tougher score to reckon with.

Never mind that he loved the idea of what he would accomplish by this point during the campaign. ("Just think of what we can accomplish in the first 100 days," he told crowds.)

Now he refers to it as a "ridiculous standard."

It may well be, but it's been used to measure every president since Franklin Delano Roosevelt -- and the White House knows it, as it prepares lists of accomplishments, charts and a rally to sing their own praises.

We sat down with two of the president's biographers to ask those who know Trump well about what they have watched as the 100-day marker approaches. Their responses have been lightly edited for style and clarity.

On the concept of the first 100 days:

Michael D'Antonio, author of "The Truth About Trump": He is not a man who is really willing to let other people determine the standards by which he'll be judged. He doesn't like for there to be an umpire calling balls and strikes. This is why he doesn't like the press. ... But he is aware of what the real score is and he is aware that now we are getting to the 100-day mark that presidents have been judged by.

On whether presidency has changed Donald Trump:

D'Antonio: I didn't think the presidency had changed Donald Trump until he acted in response to the gas attack in Syria. When I saw him do that, I realized that he might have been acting out of a certain sense of moral outrage. I think that is something new for Donald Trump. In the past, his outrages had been personal ...

Timothy O'Brien, author of "Trump Nation": We're really early on in this. ... I don't think someone who is about to turn 71 years old and has been the same person since he was 4 years old is going to get changed overnight by the presidency.

On the role of Trump's daughter Ivanka and son-in-law Jared Kushner:

D'Antonio: Loyalty is the first priority. And [Donald Trump] knows that in the case of Jared Kushner and Ivanka Trump that he has two advisers for whom there is no other priority but the success of this presidency. Others may have ambitions for themselves and ambitions in the future. But Trump can count on Jared and Ivanka serving him in the way that he prefers and putting his presidency first.

O'Brien: At the end of the day, he'll always come back to family. It's true of his business life. That's how the Trump organization was run -- it was a little boutique business, populated with family members or quasi-family members. And he's trying to replicate that in the White House... He just doesn't trust people.

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