Appendix V: Common Names For Schedule Date Constraints and Their Effects
The names for specific date constraints differ in scheduling software. In this guide, we refer to date constraint names as defined by the extensible markup language (XML) schemas used by the DOD Performance Assessments and Root Cause Analyses (PARCA) office to standardize its collection of cost and schedule data. These data are collected in support of Integrated Program Management Report (IPMR) DI-MGMT-81861. The IPMR is used to collect data for measuring cost and schedule performance on DOD acquisition contracts.
Through a joint industry and government working group, DOD PARCA has developed a data exchange instruction (DEI) for the XML schema. The DOD DEI supplements the United Nations Center for Trade Facilitation and E-business (UN/CEFACT) (XML) schema 09B. UN/CEFACT XML schemas enable entities involved in the execution of a project to exchange trade, schedule, and cost data throughout the life of the project with a standardized data content framework.
Table 10 presents the DOD/UN/CEFACT standard date constraint names along with alternative names given to the constraints in common scheduling software packages. Regardless of how one chooses to refer to a date constraint, it is far more important to recognize the effects of the constraint on the calculations within the schedule network.
Table 10: Common Names for Date Constraints and Their Primary Effects
| DOD/UN/CEFACT standard | Alternative name | Primary effect |
|---|---|---|
| Finish no earlier than (FNET) |
Early finish Finish on or after Finish not earlier than |
Forward pass: if necessary, sets the early dates of the activity so that the early finish equals the constraint date |
| Finish no later than (FNLT) |
Late finish Finish on or before Finish not later than |
Backward pass: if necessary, sets the late dates of the activity so that the late finish equals the constraint date |
| Start no earlier than (SNET) |
Early start Start on or after Start not earlier than |
Forward pass: if necessary, sets the early dates of the activity so that the early start equals the constraint date |
| Start no later than (SNLT) |
Late start Start on or before Start not later than |
Backward pass: if necessary, sets the late dates of the activity so that the late start equals the constraint date |
| Must start on (MSON) |
MSO Mandatory start |
Always sets both early and late start dates equal to the constraint date |
| Must finish on (MFON) |
MFO Mandatory finish |
Always sets both early and late finish dates equal to the constraint date |
| (Not used) | As soon as possible | The default for forward scheduling; sets the early start date as early as possible |
| (Not used) | As late as possible | The default for backward scheduling; sets the early finish date as late as possible |
Source: GAO analysis of NDIA Joint Industry/Government UN/CEFACT XML Working Group and DOD information. | GAO-16-89G
As soon as possible (ASAP) and as late as possible (ALAP) date constraints are not included in the DOD/UN/CEFACT standard but can be considered standard date constraints. ASAP date constraints are simply the default situation for forward scheduling, and ALAP date constraints are the default situation for backward scheduling. ALAP date constraints can be used in forward scheduling to force the activity to begin as late as possible. Their use in this case is rare, however, because the ALAP constraint immediately eliminates all available float for the activity. Depending on the scheduling software, the ALAP constraint may eliminate all available total float of successor activities as well.
In addition, table 10 does not include software-specific date constraints. For example, some scheduling software lets schedulers set an activity’s early and late finish dates equal while still permitting the activity to have a planned finish later than the constraint date. This allows the network to calculate negative total float on sequences of activities without actually constraining dates. These date constraint features differ by software, and users should understand the effects of particular constraints on a schedule network.