SharePoint Online Date Calculation

If you are using the New experience interface than you can achieve it with JSON column formatting.

Here is the example: 

{
"$schema": "http://columnformatting.sharepointpnp.com/columnFormattingSchema.json",
"elmType": "div",
"txtContent": {
"operator": "/",
"operands": [
{
"operator": "-",
"operands": [
{
"operator": "Number()",
"operands": [
"@now"
]
},
{
"operator": "Number()",
"operands": [
"[$DueDate]"
]
}
]
},
86400000
]
}
}

You can see in the example that it converts "@now" and "[$DueDate]" into numbers, then subtract them. Finally, the subtraction result is divided by 86400000 - the number of milliseconds in a day.

Cheers

Although Pavel Sheludkov already provided an answer, I just wanted to mention that view and column formatting now accept formulas that significantly shorten the expression.

In this case you would just write:

"=(Number([$DueDate])-Number(@now))/86400000"

The problem is SharePoint Online treats that as a calculation, and calculations are only refreshed when the item is modified in any way (at least, out of the box). The only way I could find to force the date to recalculate was to create a loop in SharePoint Designer or Flow to overwrite something in an item with its current value until due date = today, which technically changes nothing about the item while still counting as a modification, resulting in a refresh of a date difference calculation. It's an expensive workaround, especially on large lists with lots of due date items, but it's the only non-custom way I know of to manage this.