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Dates, and Time

Introduction

PLT Scheme provides basic, but important ways of manipulating time and dates. (current-seconds) returns the current time in seconds, and it is always an exact integer based on a platform-specific starting date, or epoch.

The value of (current-seconds) increases as time passes (increasing by 1 for each second that passes), and since people are not used to working with seconds, PLT provides (seconds->date n) which takes n, a platform-specific time in seconds (as returned by current-seconds), and returns an instance of the date structure type, which has the following fields:

• second : 0 to 61 (60 and 61 are for unusual leap-seconds)
• minute : 0 to 59
• hour : 0 to 23
• day : 1 to 31
• month : 1 to 12
• year : e.g., 1996
• week-day : 0 (Sunday) to 6 (Saturday)
• year-day : 0 to 365 (364 in non-leap years)
• dst? : #t (daylight savings time) or #f
• time-zone-offset : the number of seconds east of GMT for this

So, for instance printing the current year is as simple as:

(let* ((seconds (current-seconds))
       (today (seconds->date seconds)))
  (display (date-year today))
  (newline))

PLT Scheme also includes a Dates library, date.ss, which is part of Mzlib collection, and as part of the SRFI collection, PLT supports SRFI 19, Time Data Types and Procedures, which is a very complete library of date, and time related procedures.

We will discuss these libraries in the following recipes.

Recipes

• Finding Today's Date (Recipe)
• Converting DMYHMS to Seconds (Recipe)
• Converting Epoch Seconds to DMYHMS (Recipe)
• Time Arithmetic (Recipe)
• Writing and Parsing RFC 822 Dates and Times (Recipe)
• Converting Between struct:time and ISO-8601 Format (Recipe)
• Week of the Year, and Day of the Week/Month/Year (Recipe)

Comments

Contributors

-- FranciscoSolsona - 28 Apr 2004

• Set ALLOWTOPICCHANGE = SchematicsEditorsGroup