Leap for Leap Seconds!
R, 78 75 bytes

Built-ins, you say? Well...



message(paste(as.Date(.leap.seconds)-1,"23:59:60\n"),"2015-06-30 23:59:60")


R has an automatic variable .leap.seconds which contains the date and time of each leap second insertion, given in the system's local time. As of R version 3.2.0, this does not include today, so I've added that manually.

Ungolfed + explanation:

# Convert the datetime values to UTC dates. These will be a day past the
# expected output, so we can subtract 1 to get what we want.
dates &lt;- as.Date(.leap.second) - 1

# Paste the UTC time and a newline onto the end of each date
datetimes &lt;- paste(dates, "23:59:60\n")

# Print each time, including today, on its own line
message(datetimes, "2015-06-30 23:59:60")


You can try it online!