In project management a variance is the difference between a planned cost and the actual cost incurred and, normally, this leads to corrective actions. A more general definition of Variance Analysis can be found in Wikipedia and in many other web sites that specialize in this topic.
At Time Now:
Ei = Initial Estimate (Planned or Estimated Budget)
Actuals = A = Actual amount incurred
Et-c = Estimate To Complete
Ea-c = Estimate At Completion = A + Et-c
Va-c = Variance At Completion = Ei - Ea-c
At New Time (Time Now + T):
If the variance is high (too much error or discrepancy), then it will be appropriate to assign Ea-c as the new Ei.
Ei (new time) = Ea-c (time now)