Lag and Lead Time in the Forward and Backward Pass
You might recall that lag time adds time to the schedule and lead time subtracts it. An example of lag time is waiting until the glue dries before painting a model. An example of lead time is doing two activities in parallel rather than in sequence to save time in a development project. You must take lag time and lead time in a schedule into account when calculating using the forward or backward pass to calculate.
In the following example, the EF for Mix Concrete is 7. Using the forward pass, and with no lag time, that would also be the ES for Pour Concrete as well. However, if you decided to add 5 days to allow the concrete to cure (notice the arrow), the ES for Pour Concrete becomes 12. The formula is EF + lag time = ES of the next activity.
Using the backward pass in the following example, you would subtract the 5 days of lag time from the LS of Pour Concrete to get the LF of Mix Concrete.