On Saturday, President Trump will observe a milestone: his 100th day in office.
It’s also a millstone.
For nearly half a century, the marker has been used by pollsters, pundits and others who presume great wisdom to size up a fledgling White House and its still-green occupant.
Presidents of both parties have objected to the somewhat arbitrary nature of the evaluations, which, in truth, don’t always coincide with the longer view of history.
“You can use the first 100 days to try to understand some of the style of a presidency,” said Julian E. Zelizer, a Princeton scholar who has written extensively on presidents and politics. “But we really need to limit our...