Maybe this question is more PHP than Kirby, but extended googling yielded no results…
Why can’t I modify the value of a date field by a few minutes or a few days?
<?php
$safe = $page->deadline();
$safe = $safe->sub(new DateInterval('PT5M'));
$safe->modify('-10 days');
echo $safe->toDate('D, d M Y H:i:s');
?>
The blueprint is:
title: Deadline Page
preset: page
fields:
deadline:
label: Deadline
type: date
What am I missing? Thanks in advance!
Because $page->deadline() returns a Kirby Field object, not a Datetime object, and that’s what you need if you want to use sub-
So you would have to turn the value of the field into a DateTime object first.
$dateTimeObject = new DateTime($page->deadline()->value());
Reminds me that i need to add a field method to my Duration plugin, which would have made this possible straight off the field value without jumping through hoops.
Once again, texnine to the rescue! 
Thanks so much!
Now, and I don’t want to belabor the point, how could I have suspected that this was the problem here? What should be my clue?
Two things:
The PHP documentation about the DateTime object
And you could have checked what sort of information $page->deadline() returns. The easiest way to do that (without using a debugger tool) is by dumping the element
dump($page->deadline());
This would have told you that you have a Kirby\Cms\Field object and not a PHP DateTime object.
Since the DateTime class and all other PHP classes are not a part of Kirby, they are of course not documented in our documentation.
If you look closely in the docs, you will find that all Kirby methods have a documented return type, so you know what you can expect as a result of calling a method. It’s a bit special with your field names, of course, as they cannot possibly be documented. But when you call a field in your content file with $page->myFieldName() you get a Field object.
To fully understand what this stuff with classes and methods is all about, our little introduction to Object Oriented Programming with PHP might be helpful:
Excellent and thorough. Once again thanks so much!