This map is based on Eric Mullers time zone map the time zone map provided by CIA and on the date line from natural earth data.
Eric Muller's IANA time zones are merged by UTC offset (tz_world.shp). Territorial borders at sea are ignored. Instead we use the time zones as defined by CIA.
A simplified (850kb) version is provided in geojson format. mapshaper.org's visvalingam algorithm was used to acquire an 8% simplification.