The openweather API documentation states that the date format is in Unix UTC format.
https://openweathermap.org/api/one-call-3
Here's an answer on SO that describes how to convert the Unix timestamp date:
Convert unix timestamp to date in java
so your date 1427700245 is 2015-03-30 03:24:05 (GMT-04:00).
long unixSeconds = 1427700245;
// convert seconds to milliseconds
Date date = new java.util.Date(unixSeconds*1000L);
// the format of your date
SimpleDateFormat sdf = new java.text.SimpleDateFormat("yyyy-MM-dd HH:mm:ss z");
// give a timezone reference for formatting (see comment at the bottom)
sdf.setTimeZone(java.util.TimeZone.getTimeZone("GMT-4"));
String formattedDate = sdf.format(date);
System.out.println(formattedDate);
EDIT: using DateTimeFormatter which is also included in the above mentioned SO post.
final DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");
final long unixTime = 1427700245;
final String formattedDtm = Instant.ofEpochSecond(unixTime)
.atZone(ZoneId.of("GMT-4"))
.format(formatter);
System.out.println(formattedDtm); // => 2015-03-30 03:24:05