Early Start Date (ES)

The Early Start Date (ES) is the earliest possible point in time when the uncompleted portions of a schedule activity can start, based on the schedule network logic, the data date, and any schedule constraints. It is determined using the Critical Path Method (CPM) through a forward pass calculation.

Key Aspects of the Early Start Date

• Determines the Earliest Task Start – Accounts for dependencies and constraints.
• Calculated Using the Forward Pass – Helps establish the project timeline.
• Affects the Overall Project Schedule – Delays in ES impact the completion date.
• Works in Conjunction with Early Finish Date – Defines the start of a task while EF defines the earliest completion.

Early Start Calculation

The formula for determining Early Start (ES) is:

$$ES=EarliestFinishofPredecessor+1$$

Where:

• ES = Early Start Date
• EF (Earliest Finish of Predecessor) = The earliest possible finish date of the preceding task.

Example Calculation

Activity	Duration (Days)	Early Start (ES)	Early Finish (EF)
Task A	3	Day 1	Day 3
Task B	5	Day 4	Day 8
Task C	4	Day 9	Day 12

For Task A:

$$ES=1$$

For Task B (which starts right after Task A finishes):

$$ES=3+1=4$$

Mermaid Diagram: Early Start in a Project Schedule

graph TD;
    A["Task A (ES: Day 1, EF: Day 3)"] --> B["Task B (ES: Day 4, EF: Day 8)"]
    B --> C["Task C (ES: Day 9, EF: Day 12)"]

Example Scenarios

Software Development

A feature implementation task must begin early to align with testing schedules. The early start date determines resource allocation.

Construction Project

A steel frame installation must start as early as possible after foundation work is complete to prevent project delays.

Event Planning

The early start date for vendor setup ensures that staging and decorations are completed before rehearsals.

Why Early Start Date Matters

• Defines Project Scheduling Logic – Ensures tasks start at the right time.
• Affects Project Deadlines – Delays in ES push back the completion date.
• Optimizes Resource Allocation – Helps ensure teams and materials are available.
• Supports Critical Path Analysis – Helps determine scheduling flexibility.

See also: Early Finish Date (EF), Late Start Date (LS), Late Finish Date (LF), Schedule Network Analysis, Critical Path Method (CPM).