Add Determining The Age Of Things as a postgres til

postgres/determining-the-age-of-things.md

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+# Determining The Age Of Things
+
+In PostgreSQL, we can determine the age of something (or someone) by passing
+a timestamp to the `age` function.
+
+For instance, if we want to know how long it has been since y2k, we can run
+the following query:
+
+```sql
+> select age(timestamp '2000-01-01');
+           age
+-------------------------
+ 16 years 4 mons 12 days
+```
+
+Additionally, if we want to know the amount of time between two dates, we
+can pass two timestamps to the `age` function.
+
+For example, we can find out how old Prince lived to be by passing in the
+date of death and then date of birth:
+
+```sql
+> select age(timestamp 'April 21, 2016', timestamp 'June 7, 1958');
+           age
+--------------------------
+ 57 years 10 mons 14 days
+```