Better late than never answering this, right?
The part after the @ is a snap-to specifier that causes the search to start at the nearest value in that time unit. For example, if the time is 3:16:20, "-15m@m" will search from 3:01:00, where "-15m" will search from 3:01:20
Hi
it’s just like @P_vandereerden said. You should read more about it from https://docs.splunk.com/Documentation/Splunk/9.2.1/SearchReference/SearchTimeModifiers#Specify_a_sna...
r. Ismo