The paper provides a conceptual analysis of deadlines, represented in Temporal Modal Defeasible Logic. The typology is based on the following parameters: kind of deontic operator, maintenance or achievement, presence of explicit sanctions, and persistence after the deadline. The adequacy of the typology is validated against a case study of a rental agreement

Governatori, Guido

Hulstijn, Joris

Riveret, Regis

Rotolo, Antonino

Hulstijn, J.

Governatori, G.

Riveret, R.

Rotolo, A.

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On the Representation of Deadlines in a Rental Agreement

University of Queensland eSpace

Legal Knowledge and Information systems
A.R. Lodder and L. Mommers (eds)
IOS Press, 2007
167
On the Representation of Deadlines
in a Rental Agreement
Guido Governatori a and Joris Hulstijn b,1 and Re´gis Riveret c and Antonino Rotolo c
aSchool of ITEE, The University of Queensland, Australia
bVrije Universiteit, Amsterdam, The Netherlands
cCIRSFID, University of Bologna, Italy
Many agreements and contracts contain deadlines. Because deadlines combine time and
obligations, they are naturally studied by a combination of temporal logic with deontic
logic [2]. A deadline, that φ should occur before δ , can be expressed as a formula which
involves the ‘until’-operator: OBL(φUδ ). However, such a characterisation does not in-
dicate what will happen after the deadline. Since deadlines can have different functions,
it is likely that several notions of deadlines can be distinguished. Recently, we have pre-
sented a conceptual analysis of deadlines, based on the following parameters: whether
we are dealing with an achievement or maintenance obligation. persistence of the obli-
gation after the deadline, the choice of modal operators (permission or obligation) and
the presence of an explicit or implicit sanction [3].
Temporal Modal Defeasible Logic For each of these parameters template formulas are
provided, in a logic called Temporal Modal Defeasible Logic (TMDL). TMDL is an ex-
tension of Defeasible Logic [1] to capture deontic modalities (OBL, PERM) and aspects
of time, in particular intervals. For example, OBLpay; [t,max] expresses that there is an
obligation to pay from t until indefinitely. Crucial is the distinction between strict rules
(→), defeasible ruls (⇒), which allow for exceptions, and defeater rules (;), which de-
fine such possible exceptions. A superiority relation≺ specifies which rules may overrule
others, in case of conflicts. Without defeasibility it would be impossible to distinguish
exceptions from violations. Clauses may be added later, overruling previous clauses. The
superiority relation can handle conflicts between clauses from different authorities.
Analysing Deadlines The analysis is based on the following parameters: deontic op-
erators (OBLor PERM), whether we are dealing with an achievement obligation (pay
defore 1st of April), or a maintenance obligation (refrain from noisy activities between
7p.m. and 8p.m.), whether there are explicit sanctions, such as a fine, and whether the
obligation persists after the deadline. For example, the obligation to deliver a wedding
cake would not persist after the wedding.
Case Study: Rental Agreement The case study deals with temporal regulations from
a berth in a marina. We study clauses about the right and duties of yacht owners and
about payment and renewal. The contract relies on general terms and conditions from the
National Water Sports Association. Here we present an excerpt.
1Corresponding Author: dr J. Hulstijn, Faculty of Economics and Business Administration, Vrije
Universiteit, De Boelelaan 1105, 1081 HV Amsterdam, The Netherlands; E-mail: jhulstijn@feweb.vu.nl
168 Governatori et al. / Deadlines in a Rental Agreement
Article 1: Letter, Marina Hemmeland, and renter, Hulstijn, agree to the letting,
respectively rental, of berth number W156 for the summer season, which runs from
the 1st of April till the 1st of November, starting from 01/02/2007.
First, we analyse a ‘berth’ as the exclusive obligational right to have one’s yacht occupy a
certain location, indicated by number W1056. This generates an obligation for the marina
to make sure that no other yachts use the berth during the season. If not this counts as a
violation. Remember the agreement starts from the 1st of February.
agreerentermarina(W156) : 01Feb⇒ OBLmarinarenter can use(renter,W156) : [01Apr,01Nov]
OBLmarinarenter can use(renter,W156) : t,¬can use(renter,W156) : t⇒ viol(use) : t
(1)
Article 4: Payment of the rental fee must take place within 10 weekdays from receipt
of invoice, in cash at the office or by transfer to a bank account designated by letter.
Second, we analyse a ‘rental fee’ as the obligation to pay an amount of money in order to
obtain the rights mentioned above. Thus, the right to a berth is conditional on payment.
This is a standard achievement deadline formula, using a relative reference time.
agreerentermarina(W156) : 01Feb, invoice : t⇒ OBLrentermarinapay renter:[x,max]
OBLrentermarinapay:t,pay:t; ¬OBLrentermarinapay:[t+1,max]
invoice:t,OBLrentermarinapay:t+11,⇒ viol(pay):t+11
(2)
Lessons Learned Regarding coverage, we find both achievement obligations, like the
renter’s obligation to pay (2), and maintenance obligations, like (1). Most obligations
have explicit sanctions, except for harbour rules regarding noise, waste and safety. Here,
the implicit sanction is removal from the marina. There are no non-persistent achieve-
ment obligations, nor do we find positive permissions, but we do find prohibitions. Re-
garding the conceptual representation, we found that clauses may have several possible
representations, with a different legal meaning. For instance, there are various ways of in-
terpreting the silent renewal rule, that the contract is prolonged, unless one of the parties
has explicitly withdrawn from it. Regarding the logic, we found that strict rules (→), de-
feasible rules (⇒) and defeaters (;), proved extremely useful in modelling exceptions.
Intervals are crucial for expressing maintenance obligations.
In general, such a logic can be used to specify software, which can (i) generate all
possible ‘legal’ scenario’s compatible with the temporal norms, (ii) check if a specific
scenario or path conforms to the temporal norms, or (iii) alert users that a deadline has
been passed or is about to be passed. In our case, for example, the marina needs software
to generate reminders for customers who have not paid in time.
References
[1] G. Antoniou, D. Billington, G. Governatori, and M. J. Maher. Representation results for
defeasible logic. ACM Transactions on Computational Logic, 2(2):255–287, 2001.
[2] F. Dignum, H. Weigand and E. Verharen. Meeting the deadline: on the formal specification
of temporal deontic constraints. In Foundations of Intelligent Systems, LNAI 1079, 243–252.
Springer-Verlag, 1996.
[3] G. Governatori, J. Hulstijn, R. Riveret, and A. Rotolo. Adequacy of deadline characterisa-
tions. In Proceedings of Australian AI’07, LNAI 4830, 486–496. Springer-Verlag, 2007.


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On the Representation of Deadlines in a Rental Agreement

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