Reference to a time frame in which an event takes place can be done by verb inflection. If the time frame (past, present, future) is set by a temporal adverb, the verb inflection should correspond (yesterday he walk. ed; today he walk. s). Temporal violations by simple verbs (single, lexical verbs inflected with tense) in the present tense and with present time reference elicit a P600 effect (Baggio, 2008; Dragoy, Stowe, Bos, & Bastiaanse, 2012). However tense does not always coincide with time reference; in languages such as Dutch and English, reference to the past can be established by using the present tense in the present perfect (e.g. 'he has eaten the cake'). The current study investigates whether the P600 effects described by Dragoy et al. and Baggio are caused by tense or time reference violations of the verb. In the context of a past adverb, ERP responses to auxiliaries in present tense with either congruent past time reference or incongruent non-past time reference were compared. The findings show that the P600 effect for violations of the temporal context is caused by the time reference of the complete verb form, rather than by the tense.