उन्हाळी प्रमाणवेळ

**उन्हाळी प्रमाणवेळ** (किंवा *ग्रीष्म प्रमाणवेळ*) () ही जगातील अनेक देशांमध्ये उन्हाळ्यामधील स्थानिक प्रमाणवेळ आहे. विशेषतः शीत कटिबंधांमधील भागात उन्हाळ्यातील जास्त काळ टिकणाऱ्या सूर्यप्रकाशाचा फायदा घेण्यासाठी दरवर्षी वसंत ऋतूमध्ये स्थानिक वेळ एक तास पुढे ढकलली जाते व उन्हाळा संपल्यानंतर साधारण शरद ऋतूमध्ये ही वेळ एक तास मागे केली जाते. जॉर्ज व्हरनॉन हडसन ह्या न्यू झीलंडच्या शास्त्रज्ञाने १८९५ साली उन्हाळी प्रमाणवेळेची संकल्पना मांडली.

युरोप व उत्तर अमेरिकेतील बहुसंख्य देश उन्हाळी प्रमाणवेळ वापरतात.
ात उन्हाळी प्रमाणवेळ सुरू झाल्यावर घड्याळे २:०० वरून ३:०० वर ढकलली जातात.]]

उन्हाळी प्रमाणवेळेच्या वापराबाबत तज्‍ज्ञांमध्ये दुमत आहे. उन्हाळ्यात घड्याळ पुढे केल्यामुळे ऊर्जेची बचत होते असा समर्थकांचा दावा आहे तर शेतकऱ्यांना ह्या पद्धतीचा त्रास होतो अशी विरोधकांची भूमिका आहे.

## Origin 
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G.V. Hudson invented modern DST, proposing it first in 1895.

[thumb|upright=0.67|alt=Diagram of a clock showing a transition from 2:00 to 3:00.|When DST starts in [central Europe, clocks advance from 02:00 CET to 03:00 CEST.]]
thumb|upright=0.67|alt=Diagram of a clock showing a transition from 3:00 to 2:00.|When DST ends in central Europe, clocks retreat from 03:00 CEST to 02:00 CET. Other regions switch at different times.
thumb|left|alt=Time graph. The horizontal axis shows dates in 2008. The vertical axis shows the UTC offsets of eastern Brazil and eastern U.S. The difference between the two starts at 3 hours, then goes to 2 hours on February 17 at 24:00 Brazil eastern time, then goes to 1 hour on March 9 at 02:00 U.S. eastern time.|In early 2008 central Brazil was one, two, or three hours ahead of eastern U.S., depending on the date.

## Benefits and drawbacks 
[thumb|right|upright|alt=A standing man in three-piece suit, facing camera. He is about 60 and is bald with a mustache. His left hand is in his pants pocket, and his right hand is in front of his chest, holding his pocket watch.|[William Willett independently proposed DST in 1907 and advocated it tirelessly.]]

[upright=1.67|thumb|alt=Graph of sunrise and sunset times for 2007. The horizontal axis is the date; the vertical axis is the times of sunset and sunrise. There is a bulge in the center during summer, when sunrise is early and sunset late. There are step functions in spring and fall, when DST starts and stops.|Clock shifts affecting apparent sunrise and sunset times at [Greenwich in 2007.]]

[thumb|upright|alt=A standing stone in a grassy field surrounded by trees. The stone contains a vertical sundial centered on 1 o'clock, and is inscribed "HORAS NON NUMERO NISI ÆSTIVAS" and "SUMMER TIME ACT 1925".|The [William Willett Memorial Sundial is always on DST.]]

[thumb|upright|alt=Poster titled "VICTORY! CONGRESS PASSES DAYLIGHT SAVING BILL" showing Uncle Sam turning a clock to daylight saving time as a clock-headed figure throws his hat in the air. The clock face of the figure reads "ONE HOUR OF EXTRA DAYLIGHT". The bottom caption says "Get Your Hoe Ready!"|Retailers generally favor DST. [United Cigar Stores hailed a 1918 DST bill.]]
[thumb|upright|alt=Strong man in sandals and with shaggy hair, facing away from artist, grabbing a hand of a clock bigger than he is and forcing it backwards. The clock uses Roman numerals and the man is dressed in stripped-down Roman gladiator style. The text says "You can't stop time... But you can turn it back one hour at 2 a.m. on Oct. 28 when daylight-saving time ends and standard time begins."|A 2001 U.S. [Public service advertising|public service advertisement reminded people to adjust clocks manually.]]

## बाह्य दुवे 
* उन्हाळी प्रमाणवेळेचा इतिहास 

## संदर्भ आणि नोंदी 
वर्ग:प्रमाणवेळा