Pass 'interval' value to date_trunc function in PostgreSQL procedure

I am fetching the **"age"** of **two timestamp columns** (**End and Start**) and further choosing it as **to_char 'MM'** format. So I have the difference of the months from two timestamp columns. The syntax in the above statement goes like : i = (select to_char(age(End,Start),'MM')); interval_value = (select i || ' month'); **Also tried:** interval_value = i || ' Month' Now, Passing the value of 'i' in another function **date_trunc** of PostgreSQL. xyz = (select date_trunc('month',Start::date) + interval_value::text::interval); The data types for the above variables are: i numeric :=0 xyz date; interval_value varchar; But it doesn't seem to work. Is there any alternate approach I could use here. The main idea is to get the difference from two timestamps in months and then further passing the difference into the date_trunc function.