GeoDLL supports the development of geodetic software by providing geodetic functions, which are put together into function groups for specific tasks. To use Time Zone calculations in own developments a license for the function group "Time Zone Calculations" can be purchased.
This GeoDLL functions group contains functions to calculate Time Zones, UTC, DST, daylight change dates and other Time Zone parameters from coordinates or from GeoDLL Time Zone indexes.
A Time Zone is a region that has an uniform standard time for legal, commercial, and social purposes. Time Zones are primarily longitude dependent, but secondary they follow the boundaries of countries or their administrative subdivisions. The Time Zones supported by GeoDLL are based on an uniform naming convention, designed by Paul Eggert, such as America/New_York and Europe/Paris. The Time Zones have offsets from Coordinated Universal Time (UTC) as a number of hours (UTC-12 to UTC+12). Many higher latitude countries use Daylight Saving Time (DST) for a part of the year, typically by changing clocks one hour forward.
The function gettznumbycoordexact() determines a GeoDLL Time Zone index of a Time Zone from a geographic coordinate in WGS84 datum (Longitude, Latitude). In comparison with the function gettznumbycoordfast(), the function gettznumbycoordexact() is slower, but it determines the Time Zone in the border areas of the Time Zone surfaces with high precision.
For this purpose, a shape file is used, in which the geometries of all Time Zones are contained. Before the first function call, the shape file must be initialized using the function settzshapefile(). You can download the GeoDLL supported shape file named GEODLLTZ.SHP here as ZIP compressed file.
Download Time Zone shape file GEODLLTZ.SHP
The function gettznumbycoordfast() determines a GeoDLL Time Zone index of a Time Zone from a geographic coordinate in WGS84 datum (Longitude, Latitude). In comparison with the function gettznumbycoordexact(), the function gettznumbycoordfast() is considerable faster, but it determines the Time Zone in the border areas of the Time Zone surfaces only with a precision of 0,1 degree.
For this purpose, a grid file is used, which contains a geometric grid of the Time Zones. Before the first function call, the grid file must be initialized using the function settzgrid file(). You can download the GeoDLL supported grid file named GEODLLTZ.GRD here as ZIP compressed file.
Download Time Zone grid file GEODLLTZ.GRD
Detailed descriptions of all geodetic functions of this function group you find here in the online manual!