This study investigates the impact of changing the time allowance for the writing component of a diagnostic English language assessment administered on a voluntary basis to first year undergraduates at two universities with large populations of immigrant and international students following their admission to the university. The test is diagnostic in the sense of identifying areas where students may have difficulties and therefore benefit from targeted English language intervention concurrently with their academic studies. A change in the time allocation for the writing component of this assessment (from 55-30 minutes) was introduced in 2006 for practical reasons. It was believed by those responsible for implementing the assessment that a reduced time frame would minimize the problems associated with scheduling the test and accordingly encourage faculties to adopt the assessment tool as a means of identifying their students' language learning needs. The current study aims to explore how the shorter time allowance would influence the validity, reliability, and overall fairness of an EAP writing assessment as a diagnostic tool. (Contains 4 tables.)