It's pretty unlikely to be a median; if 20% of machines are 30 minutes off and 24 minutes is a median, then 30% of machines are between 24-30 minutes off, which

It's pretty unlikely to be a median; if 20% of machines are 30 minutes off and 24 minutes is a median, then 30% of machines are between 24-30 minutes off, which would seem to be a strangely narrow band. More likely, the 24 minutes is a mean average after excluding outliers. One of the standard ways to compute outliers is to measure the inter-quartile range---the "distance" between the 25th percentile value and the 75th percentile value---and call everything that is more than 1.5*IQR past the 75th percentile an outlier. You can do this without computing a mean first, and you can use it to exclude values before computing a modified mean.