public static long midnightUTC ( long epochMillis ) { Calendar day = Calendar . getInstance ( UTC ) ; day . setTimeInMillis ( epochMillis ) ; day . set ( Calendar . MILLISECOND , 0 ) ; day . set ( Calendar . SECOND , 0 ) ; day . set ( Calendar . MINUTE , 0 ) ; day . set ( Calendar . HOUR_OF_DAY , 0 ) ; return day . getTimeInMillis ( ) ; }
For bucketed data floored to the day . For example dependency links .