This is quite tricky with bundled services and fixed fee engagements as we use the Job-Task ie 'Annual Accounts - Preparation' to identify if something is done or not; not the job start/due/completion date. It's not much easier for people who invoice in arrears on a job-basis.
Job Tasks have the following date fields available:
* Job Task Start Date
* Job Task Due Date
* Job Task Completion Date (the day the box gets ticked)
We also capture in the Link Reporting Open Jobs report only the date time was last entered on a Job Task. Note however that this is '-' or zero when no time is yet entered on a future task.
To measure job-task turnaround time we would need to use some combination of the above in order to arrive at days. Something to think about is what to do when a job-task is completed prior to the job-task start date. Do job-task-days go negative? Negative numbers to funny things to averages so they might be best excluded. If someone is working 'ahead' of the work scheduled for them this is either really good, or at the detriment of another job.
Job Task Start and Due dates are typically at the beginning and end of the month so this isn't always when our team are able to start that particular job-task. We could instead measure job-task first timesheet date and job-task last timesheet date to encourage team members to have all the information required before starting a job. But this would be a low positive number (good) even when the job-task was late (bad).
A happy medium might be the date of the first timesheet entry on a job-task and the date the job-task was completed. Unfortunately 'date first timesheet entry' is not an available field in XPM job-task table, it can be derived however from the time table.