Converting epoch to string in format yyyy-mm-dd, or maybe it is not necessary

[zljubisic at gmail.com]

I have a dataframe with epoh dates, something like this:

df = pd.DataFrame( { 'epoch' : [1493928008, 1493928067, 1493928127, 1493928310, 1493928428, 1493928547]}) 

I want to create a new column with epoch converted to yyyy-mm-dd as string.

Actually, I have a epoch column, and I would like to use it for grouping data by day, let's say min, max... mayge there is no need for conversion at all.

How to do that?