The time measurement units of these calendars utilize Zero-based numbering whereby the first element each unit is a zero, or rather (keeping with Zero-based numbering terminology), the "zeroth" element of each unit is a zero. A familiar example of this is a standard 24-hour clock whereby midnight is written as 00:00:00. The gregorian calendar does not use Zero-based numbering for it's days and months, but if a slightly modified gregorian calendar utilized Zero-based numbering, new years day in 2014 could be written as 2014-0-0 instead of 2014-1-1, and the last day of 2014 could be written as 2014-11-30 instead of 2014-12-31. Notice that just because these Zero-based numbered months and days end with 11 & 30, there are still effectively a total of 12 months and 31 days because we are counting up from 0 instead of 1.
Zero-based numbering calendar systems generally have simpler algorithms making them extremely easy to calculate and are prefered by the mathematicians, statisticians, and computer programmers who are the ones responsible for carrying out large scale calculations and building the programs & apps that utilize these calendars.
The concept of zero is fairly new to the western world and has only been heavily used for a few centuries. Because of this, Zero-based numbering is often very confusing to those not familiar with it and can create some confusion amongst it's users.