Mitchell Marks and Kristian J. Hammond, A review of James F. Allen, Henry A. Kautz, Richard N. Pelavin, and Josh D. Tenenberg, Reasoning about plans, Morgan Kaufmann, San Mateo, CA, 1991; ACM SIGART (Special Interest Group in Artificial Intelligence) Bulletin, 1993, Vol. 4, No. 2, pp. 8--11.
The book that Marks and Hammond review describes the use of intervals for planning. Marks and Hammond emphasize that the interval approach to planning (originated from the works of Allen) is based on the assumption that expert knowledge about time is consistent. If expert specifications turn out to be inconsistent, then we must inform the experts about this inconsistency and ask them to correct their specifications. This tedious procedure is absolutely necessary in critical situations, when every requirement is absolutely necessary (hard). In real life, however, many requirements are soft (recommendations rather than requirements), so inconsistency and the resulting impossibility to satisfy all the requirements may be quite admissible.