The late start date (LS) is the latest point at which a schedule activity can begin without pushing out the project finish date or breaking a schedule constraint. It is one of the four dates the critical path method calculates for every activity, next to early start, early finish and late finish.
How it is calculated
Late dates come from the backward pass. You start at the project end, take the latest allowable finish of each activity, and work back through the network. The PMBOK Guide, Eighth Edition, gives the arithmetic with an example: an activity that must finish no later than day 10 and has a duration of 4 days has a late start of day 7, calculated as LS = LF – duration + 1. The “+ 1” is there because both the start day and the finish day count as working days.
What it tells you in practice
The late start is the end of the activity’s window. The early start, from the forward pass, is the beginning. The gap between the two is the total float: how long the activity can slip without delaying the project. An activity whose late start equals its early start has zero float and sits on the critical path.
Two habits follow from that. First, do not treat the late start as the planned start. Starting everything at its late date leaves no float anywhere and turns every activity into a risk to the finish date. Second, watch activities whose late start is close. They are the ones where a small delay upstream becomes a project delay.
The dates are also model output, not commitments. The eighth edition says explicitly that early and late dates do not represent the actual project schedule but define time windows within which each activity may be executed, based on durations, dependencies, leads, lags and constraints.
Late start in the PMBOK Guide 8th Edition
The eighth edition does not carry “late start date” as a separate glossary entry. The concept sits inside the definition of the critical path method (Section 5, schedule network analysis), which names early start, early finish, late start and late finish as the results of the forward and backward pass. The idea is unchanged from earlier editions; only the standalone definition is gone. Older material that quotes “late start date” from the third or fourth edition is describing the same calculation.
FAQ
Is the late start date the same as the deadline for starting an activity?
Only if nothing else changes. It is the latest start that keeps the current finish date, given the current logic and constraints. If durations or dependencies change, the late start changes with them.
What if the late start is earlier than the early start?
Then the float is negative. Constraints are forcing dates earlier than the logic and durations allow. The eighth edition describes negative float analysis as a way to explore acceleration strategies, for example compressing durations or changing logical relationships.
How does late start relate to free float?
Total float is measured against the project finish or a constraint, using late dates. Free float is measured against the early start of the next activity. An activity can have total float and no free float at the same time.
By Tom, PMP-certified since 2004. Last updated: September 2026.