Store a temporal value from an expression as variable

[saig0]

Is your feature request related to a problem? Please describe.
In a process, I need to calculate a temporal value (i.e. date, time, date-time, or duration) in an expression and store it in a variable. Later, the variable may be used by a timer event.

Currently, the temporal value is not serialized as a variable. Instead, it replaces the value with null.

As a workaround, I need to transform the value in the expression into a string explicitly.
This way is also mentioned in the docs: https://docs.camunda.io/docs/product-manuals/concepts/expressions#temporal-expressions

However, it is easy to forget to transform the value and it's hard to root cause the problem.

Describe the solution you'd like
A temporal value is stored as a string variable by default.

The string value can be used in a FEEL conversion function to restore the temporal value.

Describe alternatives you've considered
Native support for temporal values without string serialization. But JSON doesn't have a type for these values.

Additional context

[Zelldon]

Example https://github.com/zeebe-io/zeebe-cluster-testbench/blob/develop/processes/actionRunner.bpmn#L8 where this would be useful. Or in other words where I run into this problem. The timeout was set to null, when I used the duration function and I didn't know why.