Field Automation Based On Date Field

I've created a board with the fields "Start Date," "End Date," and "Deadline." The "Deadline" represents the number of days required to complete the task.

While I currently enter the "Start Date" and "End Date" manually, I would like to automate the calculation of the "Deadline" based on these dates. Is there a way to achieve this?

I would like to do it without using Power Automate if possible.

I've checked the available configurations and searched both here and on Google, but haven't found a solution yet.

I am afraid that there is no out-of-box method can automatically calculate of the Deadline field based on Start Date and End Date.

Since you don't want to use Power Automate, I suggest that you can use Script to run Rest API to calculate date.

Refer to the following steps:

Step1: Create a boards query(Boards -> Queries) to list all work items that need to be calculated.

For example:

You can record the QueryID in the url.

Step2: We can use PowerShell script to run the following Rest APIs:

Wiql - Query By Id: Use to list work items in Query.

Work Items - Get Work Item : Use to get the value of the Start Date and End Date field.

Work Items - Update: Use to update the DeadLine field in work item.

Here is the PowerShell sample:

$token = "PAT"

$Boardsqueryid ="QueryId"

$ProjectId = "Projectname"

$OrganizationId = "OrganizationName"

$token = [System.Convert]::ToBase64String([System.Text.Encoding]::ASCII.GetBytes(":$($token)"))

$getworkitems ="https://dev.azure.com/$($OrganizationId)/$($ProjectId)/_apis/wit/wiql/addf177b-a5f8-460a-a976-40a5b52f2923?api-version=7.1-preview.2"

$response2 = Invoke-RestMethod -Uri $getworkitems -Headers @{Authorization = "Basic $token"} -Method Get  -ContentType application/json

foreach($workitem in $response2.workitems)
{
    $workitemid =  $workitem.id

    echo "workitemid: $workitemid"

    $url="https://dev.azure.com/$($OrganizationId)/$($ProjectId)/_apis/wit/workitems/$($workitemid)?api-version=7.1-preview.3"

    $response = Invoke-RestMethod -Uri $url -Headers @{Authorization = "Basic $token"} -Method Get  -ContentType application/json

    $startdate = $response.fields.'Microsoft.VSTS.Scheduling.StartDate'
    
     echo "startdate: $startdate"

    $enddate = $response.fields.'Custom.EndDate'
     
    echo "enddate: $enddate"

    $ts = New-TimeSpan -Start $startdate -End $enddate

    $days = $ts.Days
    echo "deadlinedays: $days"

    $urlupdateworkitem = "https://dev.azure.com/$($OrganizationId)/$($ProjectId)/_apis/wit/workitems/$($workitemid)?api-version=7.1-preview.3"

     $body="[
              {
                `"op`": `"add`",
                `"path`": `"/fields/Custom.deadline`",
                `"value`": `"$days`"
              }

           ]"


  $response1 = Invoke-RestMethod -Uri $url -Headers @{Authorization = "Basic $token"} -Method PATCH  -Body $body -ContentType application/json-patch+json
}

Result:

Powershell side:

Work item Page: