Support interval field values with fractional parts

In PostgreSQL, field values can have fractional parts; for example '1.5 week' or '01:02:03.45'. Such input is converted to the appropriate number of months, days, and seconds for storage. When this would result in a fractional number of months or days, the fraction is added to the lower-order fields using the conversion factors 1 month = 30 days and 1 day = 24 hours. For example, '1.5 month' becomes 1 month and 15 days. Only seconds will ever be shown as fractional on output. See https://www.postgresql.org/docs/current/datatype-datetime.html#DATATYPE-INTERVAL-INPUT

For example:

maxim=# SELECT INTERVAL '1.5 months' AS "One month 15 days";
 One month 15 days 
-------------------
 1 mon 15 days
(1 row)