dates lose timezone attribute when you subset them

[limnoliver]

@rbcarvin learned something new with this one! In RMevents, we find "start date" by subsetting by event and finding the minimum. When you subset dates like this, you drop the attributes of the data, which includes time zone. Then, R assigns a time zone, which is the time zone of your computer. So, this is why we're getting converted dates that are correct, but events that are an hour off during DST.

[rbcarvin]

It looks like this same issue is occurring with RMevents_sample. @limnoliver