The "unreadable format" you have posted is known as a UNIX timestamp. It is the number of seconds from January 1, 1970, to the date represented.
Using a handy online tool, you can quickly see what that timestamp equates to if needed: http://www.unixtimestamp.com/index.php
In order to reformat this timestamp, in your case YYYY-MM-DD, you will need to reference the PHP date format parameters: http://php.net/manual/en/function.date.php
For your example, you would want
'Y-m-d'. This assumes you want leading zeros on month and day.
Y = A full numeric representation of a year, 4 digits
m = Numeric representation of a month, with leading zeros
d = Day of the month, 2 digits with leading zeros