New system field/parameter: Date/Time of conversion to task

Explain the problem as you see it

We have several useful fields/parameters like:

${cdate} and ${ctime} - Created date and time
${mdate} and ${mtime} - Modified date and time
${sys:description} - Description field
${sys:createdAt} - Time of creation
${sys:lastModifiedAt} - Time of last edit
${sys:lastModifiedBy} - User who last edited the node
${sys:modifiedBy} - All users who ever created/modified the node
${sys:owner} - Parent node
${sys:dateFromDayNode} - The day node this node belongs to (Calendar date system field)
${sys:doneTime} - Date and time this node was marked done

A new one like ${sys:convertedToTaskTime} and ${sys:taggedAs{{CUSTOMTAGNAME}}Time} would be useful to show when a node has been converted to task being worked on – either manually by making it a task node or by adding a new tag like "#in progress".

With ${sys:doneTime} it could then be used as a time logger for tasks:

Create task as a plain node.
It's waiting for its time in some kind of backlog.
Decide to make it more actionable - tag is as "#in progress" or convert it to todo (or both).
Complete the task

Why is this a problem for you?

I have a long list of illustrations to be created. When I start working on an illustration task, I convert its node to a todo. After finishing the illustration, I complete its todo.

I would like to track the time from "start working on an illustration task" to "complete its todo".

Suggest a solution

Have functional:

${sys:convertedToTaskTime} or
${sys:taggedAs{{CUSTOMTAGNAME}}Time}
to be then later used for example as a date ranges in Calendar View.