How can I show Calendar Type Data in FreeMarker

I spent lots of time on this, it made me crazy.

In Java controller,var mycalendar is Calendar Type.Put it into model

model.addAttribute("mycalendar", mycalendar);

In Freemarker template:

${pp}

Error like this:

java.util.GregorianCalendar[time=1507864967342,areFieldsSet=true,areAllFieldsSet=true,lenient=true,zone=sun.util.calendar.ZoneInfo[id="Asia/Shanghai",offset=28800000,dstSavings=0,useDaylight=false,transitions=19,lastRule=null],firstDayOfWeek=1,minimalDaysInFirstWeek=1,ERA=1,YEAR=2017,MONTH=9,WEEK_OF_YEAR=41,WEEK_OF_MONTH=2,DAY_OF_MONTH=13,DAY_OF_YEAR=286,DAY_OF_WEEK=6,DAY_OF_WEEK_IN_MONTH=2,AM_PM=0,HOUR=11,HOUR_OF_DAY=11,MINUTE=22,SECOND=47,MILLISECOND=342,ZONE_OFFSET=28800000,DST_OFFSET=0]

How can I show this Calendar type correctly like ‘yyyy-MM-dd HH:mm:ss’