Time - (Unix|POSIX|Epoch) time

Second

It's the number of seconds (or millisecond) that have elapsed since 00:00:00 Coordinated Universal Time (UTC), Thursday, 1 January 1970, not counting leap seconds.

Because it does not handle leap seconds, it is neither a linear representation of time nor a true representation of UTC.

Instants:

• after the epoch have positive values,
• before the epoch have negative values.

Day

An epoch day is a incrementing count of days where day 0 is 1970-01-01.

Bash / Linux

Unix time may be checked on most Unix systems by typing date +%s on the command line.

Java

• What is a Java Instant? (Moment)
• javase/9/docs/api/java/lang/System.html

System.currentTimeMillis()

Oracle

• Example 1:

myDate = TO_DATE('19700101000000','YYYYMMDDHH24MISS') + NUMTODSINTERVAL(myUnixTimeStamp, 'SECOND')

• Example 2:

select TO_CHAR( FROM_TZ( CAST(DATE '1970-01-01' + (1/24/60/60/1000) * <column name> AS TIMESTAMP), 'America/New_York'), 'MM/DD/YYYY HH24:MI:SS') from <tablename>;