"Time in grade" refers to the time somebody has spent actually doing a job, studying a subject or holding a position. Time in grade can be a factor in qualifying for promotion, getting a new job or receiving a certificate. Whatever purpose it serves, calculating time in grade always follows the same basic process. Define what time in grade actually means, calculate according to your definitions and forecast when people will reach their benchmark goals.
Time in grade can be measured as the time between when somebody started a position and the present day. However, you will want to set parameters about when time counts and doesn't count. For example, when measuring time in grade for a teacher, you may want to not count a year spent on sabbatical. Similarly, you may want to arrange for some time to count extra; for example, assigning 1 1/2 years of time for each year spent in a particularly challenging assignment.
Construct a timeline for each candidate beginning on their starting date for consideration and running to the present. Subdivide appropriately according to the length of time you're dealing with (i.e., months, weeks or years). For each time subdivision, adjust candidates' time in rank scores according to the definitions you set up earlier. Record the results.
Use the timelines and scores you calculated in step two to make reasonable forecasts for when candidates will achieve needed time in grade. If you're culling candidates for a single position, this step may not be necessary. If you're making predictions for graduation from a program, you can use this information to help some candidates get extra time in to be sure they graduate with their cohort.