Just figured it out:
If you start calculating in a formula with a date/time you will get a decimal as a result:
.
This number represent the number of days since 01/01/1970.
That means today 6/18/2020 is 18431 days past 01/01/1970.
The decimal represent the fraction of time per day.
If you want to add 1 day you can just calculate
If you want to add minutes you have to use the fraction of 1/1440 for 1 minute
In the Custom Formula you can add minutes like this:
Where Input represents the number of minutes you want to add.
And don’t miss the brackets 