Subsecond Unix times around the first supported TIMESTAMP may be wrong

[localhost:21000] > select cast(-17987443201.0 as timestamp);
+-----------------------------------+
| cast(-17987443201.0 as timestamp) |
+-----------------------------------+
| NULL                              |
+-----------------------------------+
Fetched 1 row(s) in 0.01s

[localhost:21000] > select cast(-17987443200.0 as timestamp);
+-----------------------------------+
| cast(-17987443200.0 as timestamp) |
+-----------------------------------+
| 1400-01-01 00:00:00               |
+-----------------------------------+
Fetched 1 row(s) in 0.01s

[localhost:21000] > select cast(-17987443200.1 as timestamp);
+-----------------------------------+
| cast(-17987443200.1 as timestamp) |
+-----------------------------------+
| 1400-01-01 00:00:00.100000000     |
+-----------------------------------+
Fetched 1 row(s) in 0.01s

[localhost:21000] > select cast(cast(-17987443200.1 as decimal(31,1)) as timestamp);
+----------------------------------------------------------+
| cast(cast(-17987443200.1 as decimal(31,1)) as timestamp) |
+----------------------------------------------------------+
| 1400-01-01 00:00:00.100000000                            |
+----------------------------------------------------------+

mj=# set timezone to 'GMT';
SET
mj=# select to_timestamp(0);
      to_timestamp      
------------------------
 1970-01-01 00:00:00+00
(1 row)

mj=# select to_timestamp(-17987443200.1);
         to_timestamp          
-------------------------------
 1399-12-31 23:59:59.900002+00
(1 row)

mj=# select to_timestamp(-17987443200.0);
      to_timestamp      
------------------------
 1400-01-01 00:00:00+00
(1 row)

  static TimestampValue FromUnixTimeNanos(time_t unix_time, int64_t nanos) {
    boost::posix_time::ptime temp = UnixTimeToPtime(unix_time);
    temp += boost::posix_time::nanoseconds(nanos);
    return TimestampValue(temp);
  }

  /// Returns a TimestampValue where the integer part of the specified 'unix_time'
  /// specifies the number of seconds (see above), and the fractional part is converted
  /// to nanoseconds and added to the resulting TimestampValue.
  static TimestampValue FromSubsecondUnixTime(double unix_time) {
    const time_t unix_time_whole = unix_time;
    boost::posix_time::ptime temp = UnixTimeToPtime(unix_time_whole);
    temp += boost::posix_time::nanoseconds((unix_time - unix_time_whole) / ONE_BILLIONTH);
    return TimestampValue(temp);
  }