Unix Timestamp to ISO-8601 String - oracle

Question

I have an Oracle table called ACQDATA with a field READDATETIME where I store a Unix timestamp in milliseconds as an INTEGER (NUMBER(38)) type.
SQL> select READDATETIME from ACQDATA where ID=1000;
READDATETIME
____________
1.4793E+12
I need to select that value as a ISO-8601 string (YYYY-MM-DDTHH:MM:SS.mmm):
SQL> select READDATETIME from ACQDATA where ID=1000;
READDATETIME
-------------------
1.4793E+12
I´ve tried to convert it using TO_CHAR, but the result is messy:
SQL> select TO_CHAR(TO_DATE('1970-01-01','YYYY-MM-DD') + NUMTODSINTERVAL(READDATETIME, 'SECOND'), 'YYYY-MM-DD HH24:MI:SS') from ACQDATA where ID=1000;
Error at line 1:
ORA-01873: the leading precision of the interval is too small
Help appreciated.

Answer 1

Alex's answer is not fully correct. Unix timestamp is always based on 1970-01-01 00:00:00 UTC
Unless your session runs on UTC time zone the precise solution would be like this:
select
TO_CHAR((TIMESTAMP '1970-01-01 00:00:00 UTC' + readdatetime/1000 * INTERVAL '1' SECOND) AT LOCAL, 'YYYY-MM-DD"T"HH24:MI:SS.FF3')
from ACQDATA where ID=1000;
or
select
TO_CHAR((TIMESTAMP '1970-01-01 00:00:00' AT TIME ZONE 'UTC' + readdatetime/1000 * INTERVAL '1' SECOND) AT LOCAL, 'YYYY-MM-DD"T"HH24:MI:SS.FF3')
from ACQDATA where ID=1000;
or if you prefer functions instead of literals:
select
TO_CHAR((TO_TIMESTAMP_TZ('1970-01-01 00:00:00 UTC', 'YYYY-MM-DD HH24:MI:SS TZR') + numtodsinterval(readdatetime/1000, 'SECOND')) AT LOCAL, 'YYYY-MM-DD"T"HH24:MI:SS.FF3')
from ACQDATA where ID=1000;

Answer 2

Your readdatetime seems to be in milliseconds. Oracle date arithmetic works on the basis of days, so you need to convert that number to the number of days it represents; one day is 86400 seconds, so it's 86400000 milliseconds:
with acqdata (id, readdatetime) as (
select 1000, 1479318995000 from dual
)
select to_char(date '1970-01-01' + (READDATETIME/86400000), 'YYYY-MM-DD"T"HH24:MI:SS')
from ACQDATA where ID=1000;
TO_CHAR(DATE'1970-0
-------------------
2016-11-16T17:56:35
The T is added as a character literal.
SQL Developer defaults to show numbers that large in scientific notation. You can change that default with set numformat, or use to_char() to show the whole value:
select readdatetime, to_char(readdatetime, '9999999999999') as string
from ACQDATA where ID=1000;
READDATETIME STRING
------------ --------------
1.4793E+12 1479318995000
If your value has fractional seconds, so the last three digits are not zeros, you can convert the date to a timestamp and add on the fractional leftovers; this also adds the UTC 'Z' indicator for fun:
with acqdata (id, readdatetime) as (
select 1000, 1479300462063 from dual
)
select to_char(cast(date '1970-01-01' + (readdatetime/86400000) as timestamp)
+ numtodsinterval(remainder(readdatetime, 1000)/1000, 'SECOND'),
'YYYY-MM-DD"T"HH24:MI:SS.FF3"Z"')
from acqdata where id=1000;
TO_CHAR(CAST(DATE'1970-01-01'+
------------------------------
2016-11-16T12:47:42.063Z
Or without the intermediate date value, starting from a timestamp literal:
with acqdata (id, readdatetime) as (
select 1000, 1479300462063 from dual
)
select to_char(timestamp '1970-01-01 00:00:00'
+ numtodsinterval(readdatetime/1000, 'SECOND'),
'YYYY-MM-DD"T"HH24:MI:SS.FF3"Z"')
from acqdata where id=1000;
TO_CHAR(TIMESTAMP'1970-0
------------------------
2016-11-16T12:47:42.063Z
As #Wernfried ponted out, it's better to explicitly show that the epoch time is starting from UTC:
alter session set time_zone='America/New_York';
with acqdata (readdatetime) as (
select 1479300462063 from dual
union all select 1467331200000 from dual
union all select 1467648000000 from dual
)
select readdatetime,
to_char(timestamp '1970-01-01 00:00:00' + numtodsinterval(readdatetime/1000, 'SECOND'),
'YYYY-MM-DD"T"HH24:MI:SS.FF3') as implicit,
to_char(cast(timestamp '1970-01-01 00:00:00' as timestamp with time zone)
+ numtodsinterval(readdatetime/1000, 'SECOND'),
'YYYY-MM-DD"T"HH24:MI:SS.FF3TZH:TZM') as local_offset,
to_char(timestamp '1970-01-01 00:00:00 UTC' + numtodsinterval(readdatetime/1000, 'SECOND'),
'YYYY-MM-DD"T"HH24:MI:SS.FF3TZH:TZM') as utc_offset,
to_char(timestamp '1970-01-01 00:00:00 UTC' + numtodsinterval(readdatetime/1000, 'SECOND'),
'YYYY-MM-DD"T"HH24:MI:SS.FF3TZR') as utc
from acqdata;
READDATETIME IMPLICIT LOCAL_OFFSET UTC_OFFSET UTC
-------------- ----------------------- ----------------------------- ----------------------------- --------------------------
1479300462063 2016-11-16T12:47:42.063 2016-11-16T12:47:42.063-05:00 2016-11-16T12:47:42.063+00:00 2016-11-16T12:47:42.063UTC
1467331200000 2016-07-01T00:00:00.000 2016-07-01T01:00:00.000-04:00 2016-07-01T00:00:00.000+00:00 2016-07-01T00:00:00.000UTC
1467648000000 2016-07-04T16:00:00.000 2016-07-04T17:00:00.000-04:00 2016-07-04T16:00:00.000+00:00 2016-07-04T16:00:00.000UTC