This article provides an introduction to the use of altmetrics as a tool to assess research impact. In particular, it looks at the evidence behind claims that altmetrics allow the impact of research to be measured in days rather than years. Low correlations between altmetrics and article citations make it doubtful that altmetrics can reliably predict future citations. In addition, there are good reasons to qualify statements that altmetrics measure the wider impact of research on society. Librarians should be careful not to overstate the value of altmetrics when recommending their use as a complement to more traditional measures of research quality.