String DateStr="2012-05-31T13:48:04Z";
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss'Z'");
Date d=sdf.parse(DateStr);
System.out.println(d.getTime());

Instant.parse( "2012-05-31T13:48:04Z" )
       .toEpochMilli()
       / 1_000L

Instant instant = Instant.parse( "2012-05-31T13:48:04Z" ) ;

long millisSinceEpoch = instant.toEpochMilli() ;

long secondsSinceEpoch = ( millisSinceEpoch / 1_000L ) ;