Objective Time spent in the emergency department (ED) before admission to hospital is often considered an important key performance indicator (KPI). Throughout Australia and New Zealand, there is no standard definition of time of admission for patients admitted through the ED. By using data submitted to the Australian and New Zealand Intensive Care Society Adult Patient Database, the aim was to determine the differing methods used to define hospital admission time and assess how these impact on the calculation of time spent in the ED before admission to an intensive care unit (ICU). Methods Between March and December of 2010, 61 hospitals were contacted directly. Decision methods for determining time of admission to the ED were matched to 67787 patient records. Univariate and multivariate analyses were conducted to assess the relationship between decision method and the reported time spent in the ED. Results Four mechanisms of recording time of admission were identified, with time of triage being the most common (28/61 hospitals). Reported median time spent in the ED varied from 2.5 (IQR 0.83-5.35) to 5.1h (2.82-8.68), depending on the decision method. After adjusting for illness severity, hospital type and location, decision method remained a significant factor in determining measurement of ED length of stay. Conclusions Different methods are used in Australia and New Zealand to define admission time to hospital. Professional bodies, hospitals and jurisdictions should ensure standardisation of definitions for appropriate interpretation of KPIs as well as for the interpretation of studies assessing the impact of admission time to ICU from the ED. What is known about the topic? There are standards for the maximum time spent in the ED internationally, but these standards vary greatly across Australia. The definition of such a standard is critically important not only to patient care, but also in the assessment of hospital outcomes. Key performance indicators rely on quality data to improve decision-making. What does this paper add? This paper quantifies the variability of times measured and analyses why the variability exists. It also discusses the impact of this variability on assessment of outcomes and provides suggestions to improve standardisation. What are the implications for practitioners? This paper provides a clearer view on standards regarding length of stay in the ICU, highlighting the importance of key performance indicators, as well as the quality of data that underlies them. This will lead to significant changes in the way we standardise and interpret data regarding length of stay.