public static long getTopOfTheDayTimestamp(long ts) {
    return (ts - (ts % MILLIS_ONE_DAY));
  }
Returns the timestamp of that day's start timestamp (that is midnight 00:00:00 AM) for a given input timestamp.
For instance, if the supplied timestamp is 100000, this method would return 86400, which corresponds to
2 January 1970, 00:00:00 GMT.

@param ts Timestamp for which top of the day timestamp is to be found.
@return Timestamp of that day's beginning (midnight)
app/com/linkedin/drelephant/util/Utils.java