IFC Scheduling and 4D BIM Implementation

Quick Reference

Critical Warnings

• ALWAYS use ifcopenshell.api.run("sequence.*", ...) for schedule mutations. NEVER create IfcTask, IfcWorkSchedule, or IfcRelSequence directly with model.create_entity().
• ALWAYS call add_task_time BEFORE edit_task_time. The IfcTaskTime entity MUST exist before editing.
• ALWAYS use ISO 8601 strings for dates ("2026-04-01") and durations ("P5D") in IFC4+. NEVER pass Python datetime objects directly to edit_task_time.
• NEVER assign IfcTaskTime to parent/summary tasks. ONLY leaf tasks (no subtasks) receive time data.
• NEVER pass both work_schedule and parent_task to add_task. These are mutually exclusive.
• NEVER create cyclical sequence relationships. cascade_schedule will recurse infinitely.
• ALWAYS call cascade_schedule after modifying task durations or sequences. Dates do NOT propagate automatically.
• ALWAYS pass products as a list in v0.8+ relationship functions (e.g., assign_process).
• NEVER forget to create a project bootstrap (IfcProject, units, contexts) before creating schedules.

Version Differences: IFC2X3 vs IFC4+

| Feature | IFC2X3 | IFC4 / IFC4X3 |