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