4.13 Date and Time Format

There are two data types in mcal, namely date and time format. One is the date type and the other are date type. Time formatted data is represented with date formatted data as a set. The command uses date_time library of boost C + + library based on the Gregorian calendar, date type uses class boost::gregorian::date, and time type uses class boost::posix_time::ptime. For more details, refer to documentation in boost.org.

Date class is managed as a 32-bit integer internally, and supports dates ranging from January 1,1400 to December 31, 9999. Operations on date is based on the Gregorian calendar. NULL value will be returned on invalid date (for example, 2013/2/29 or 1399/12/31).

On the other hand, class ptime is managed as 64 bit. It is a time system with nano-second/micro-second resolution. The mcal command do not have an interface for time point manipulation. Class ptime is dependent on gregorian::date for the interface to the date portion of a time point, thereby enable time calculations across different dates. NULL value is returned on invalid time (e.g.18:62:11).

MCMD deals with CSV text, date/time must be assigned as character string in the data. The command then converts character string to date and time type for processing various operations.The final result converted back to character string in the output. The string format is expressed as 8-digit fixed-length string for date (e.g. "20130911"), 14-digit fixed-length string for time (e.g. "20130911110528") or the standard 6-digit fixed-length string (for example, "110528").

Figure 4.1 below shows the relationship of date type, time type and various functions.

Figure 4.1: The relationship among time type, date type and various functions using September 6, 2013 at 43 minutes, 27 seconds. The solid line box indicates actual data, the dotted line indicate the functions.

Other than using fixed length character string as a standard for date/time, user can use Julian day (e.g. continuous count of days since Julian period such as January 1, 4713 BC) or UNIX time (e.g. number of seconds that have elapsed since 00:00:00 Coordinated Universal Time (UTC), Thursday, 1 January 1970) as a signed integer to represent date and time. The command supports Julian day and UNIX time, as well as conversion functions of date and time data type.

The mcal command uses an internal date/time format which is based on the Gregorian calendar, thus the date range is limited from January 1,1400 to December 31,9999. Since UNIX time is a signed integer data type of 32 bits, the furthest time that can be represented this way is 03:14:07 UTC on Tuesday, 19 January 2038, date beyond this point will be interpreted incorrectly due to integer overflow. The drawback of using UNIX time and Julian day is that one will not be able to tell the date and time by looking at the number.