We present the results of our exploratory analysis on the relationship that exists between relevance and time. We observe how the amount of documents published in a given interval of time is related to the probability of relevance, and, using the time series analysis, we show the existence of a correlation between time and relevance. As an initial application of this analysis, we study query expansion exploiting the detection of publication time peaks over the Blog06 collection. We finally propose an effective approach for the query expansion in the blog search domain. Our approach is based on the documents publication trend being so completely independent of any external resource.