Theory of Operation
Why is there only one class Zend_Date for handling dates and times in Zend Framework?
Many languages split the handling of times and calendar dates into two classes. However, Zend Framework strives for extreme simplicity, and forcing the developer to manage different objects with different methods for times and dates becomes a burden in many situations. Since Zend_Date methods support working with ambiguous dates that might not include all parts (era, year, month, day, hour, minute, second, timezone), developers enjoy the flexibility and ease of using the same class and the same methods to perform the same manipulations (e.g. addition, subtraction, comparison, merging of date parts, etc.). Splitting the handling of these date fragments into multiple classes would create complications when smooth interoperation is desired with a small learning curve. A single class reduces code duplication for similar operations, without the need for a complex inheritance hierarchy.
All dates and times, even ambiguous ones (e.g. no year), are represented internally as absolute moments in time, represented as a UNIX timestamp expressing the difference between the desired time and January 1st, 1970 00:00:00 GMT. This was only possible, because Zend_Date is not limited to UNIX timestamps nor integer values. The BCMath extension is required to support extremely large dates outside of the range Fri, 13 Dec 1901 20:45:54 GMT to Tue, 19 Jan 2038 03:14:07 GMT. Additional, tiny math errors may arise due to the inherent limitations of float data types and rounding, unless using the BCMath extension.
Date parts as timestamp offsets
Thus, an instance object representing three hours would be expressed as three hours after January 1st, 1970 00:00:00 GMT -i.e. 0 + 3 * 60 * 60 = 10800.
Where possible, Zend_Date usually uses PHP functions to improve performance.