Date calculation

Sorry, this is PHP not C#
Try this before record added
DateTime dt1 = DateTime.Parse(values["DateEntered"].ToString()); values["NextDate"] = dt1.AddMonths(12).ToString();

It will help if you explain what exactly doesn't work right now. It doesn't insert any value into "next inspection" field or inserts an incorrect value?

No, It does not fill in any value in Next inspection

That gives : syntax error, unexpected 'dt1' (T_STRING) in line 1

// Fecha actual
$fecha_actual = date('Y-m-d');
// Sumar un año a la fecha actual
$nueva_fecha = date('Y-m-d', strtotime('+1 year', strtotime($fecha_actual)));
// Imprimir la nueva fecha
echo $nueva_fecha;

You have to start from the very beginning and provide all the relevant info.
<ol> What event do you use? Do you expect this value to appear on the page on right in your database?
</ol>

To admin:
I made one field "Createdate" and another which is"Next inspection". When I fill in the date of today (as default) in "Createdate" I want to see on the same page in the field "Next inspection" the same date but 12 months later.
Does that make sense to you?

You didn't answer the question number 1.

Sorry about that ; Question no 1: Before record added and before record edited, seems odd but I have taken that from an example given here.

Thank you.
BeforeEdit/BeforeAdd events are executed after user clicks Save button. This means that if you need something to be recalculated on the screen, in realtime, this event will not work for you.
To achive what you looking for you need to use Field Events and Javascript code:
https://xlinesoft.com/phprunner/docs/field_events.htm
Example 2 shows an example of calculation and filling another field.

But then I need a Java string and no PHP to calculate 12 months more. Who has done that already ?

Google "javascript add year to date" and it shows tons of examples.
I like this one for instance:
https://stackoverflow.com/questions/33923973/adding-1-year-to-a-date-with-javascript