Dynamic (Column Based) Interval

I usually multiply the number by interval '1 day' or similar, e.g.:

select now() + interval '1 day' * a.number_of_days from a;

I know this is a year old, but if you need to use a column to specify the actual interval (e.g. 'days', 'months', then it is worth knowing that you can also CAST your string to an Interval, giving:

SELECT now()+ CAST(the_duration||' '||the_interval AS Interval)

So the the original question would become:

SELECT now() + CAST(a.number_of_days||" DAYS" AS Interval) as "The Future Date" FROM a;

I prefer this way. I think its pretty easy and clean. In postgre you need interval to use + operator with timestamp

select (3||' seconds')::interval;

select now()+ (10||' seconds')::interval,now();

where you can use seconds,minutes...days,months... and you can replace the numbers to your column.

select now()+ (column_name||' seconds')::interval,now()
from your_table;

To creating intervals those based on column values, I recommend to add two columns in your table. For example, column "period_value"::INT4 and column "period_name"::VARCHAR. Column "period_name" can store the following values:

• microsecond
• milliseconds
• second
• minute
• hour
• day
• week
• month
• quarter
• year
• decade
• century
• millennium

+--------------+-------------+
| period_value | period_name |
+--------------+-------------+
| 2            | minute      |
+--------------+-------------+

Now you can write:

SELECT NOW() - (period_value::TEXT || ' ' || period_name::TEXT)::INTERVAL FROM table;

Use make_interval()

SELECT NOW() + make_interval(days => a.number_of_days) AS "The Future Date" 
FROM a;

But in general it might be a better idea to use a column defined as interval, then you can use any unit you want when you store a value in there.