Complement Realised As Prepositional Phrase

the only time the ceremony has been interrupted in the last 700 years	was	during the Second World War
Value/Location	Process	Token/Location
Subject	Finite	Complement

Reasoning:

If 'a Complement is an element within the Residue that has the potential of being Subject but is not' (Halliday & Matthiessen 2004: 122), and 'an Adjunct is an element that has not got the potential of being Subject' (op cit: 123), then the 'circumstance as participant' during the Second World War functions as Complement, not Adjunct, despite the fact that a Complement 'is typically realised by a nominal group' (ibid), since it has the potential of being Subject, as the following agnate clause demonstrates:

during the Second World War was the only time the ceremony has been interrupted in the last 700 years (wasn't it?)

during the Second World War	was	the only time the ceremony has been interrupted in the last 700 years
Token/Location	Process	Value/Location
Subject	Finite	Complement

See also Fig. 5-26 (op cit: 244).