Change of time zone after using setFullYear / setUTCFullYear

Question

I was experimenting with javascript dates; and as a part of it, I wrote the following code in firefox console:

var test = new Date();
for (var i = test.getFullYear(); i >= 1; i--) {
    test.setUTCFullYear(i, 0, 1);
    console.log(test);
}

This generated the following output:

Sun Jan 01 2023 08:15:24 GMT+0600 (Bangladesh Standard Time)
Sat Jan 01 2022 08:15:24 GMT+0600 (Bangladesh Standard Time)
Fri Jan 01 2021 08:15:24 GMT+0600 (Bangladesh Standard Time)
Wed Jan 01 2020 08:15:24 GMT+0600 (Bangladesh Standard Time)
Tue Jan 01 2019 08:15:24 GMT+0600 (Bangladesh Standard Time)
Mon Jan 01 2018 08:15:24 GMT+0600 (Bangladesh Standard Time)
Sun Jan 01 2017 08:15:24 GMT+0600 (Bangladesh Standard Time)
Fri Jan 01 2016 08:15:24 GMT+0600 (Bangladesh Standard Time)
Thu Jan 01 2015 08:15:24 GMT+0600 (Bangladesh Standard Time)
Wed Jan 01 2014 08:15:24 GMT+0600 (Bangladesh Standard Time)
Tue Jan 01 2013 08:15:24 GMT+0600 (Bangladesh Standard Time)
Sun Jan 01 2012 08:15:24 GMT+0600 (Bangladesh Standard Time)
Sat Jan 01 2011 08:15:24 GMT+0600 (Bangladesh Standard Time)
Fri Jan 01 2010 08:15:24 GMT+0600 (Bangladesh Standard Time)
Thu Jan 01 2009 08:15:24 GMT+0600 (Bangladesh Standard Time)
Tue Jan 01 2008 08:15:24 GMT+0600 (Bangladesh Standard Time)
Mon Jan 01 2007 08:15:24 GMT+0600 (Bangladesh Standard Time)
Sun Jan 01 2006 08:15:24 GMT+0600 (Bangladesh Standard Time)
Sat Jan 01 2005 08:15:24 GMT+0600 (Bangladesh Standard Time)
Thu Jan 01 2004 08:15:24 GMT+0600 (Bangladesh Standard Time)
Wed Jan 01 2003 08:15:24 GMT+0600 (Bangladesh Standard Time)
Tue Jan 01 2002 08:15:24 GMT+0600 (Bangladesh Standard Time)
Mon Jan 01 2001 08:15:24 GMT+0600 (Bangladesh Standard Time)
Sat Jan 01 2000 08:15:24 GMT+0600 (Bangladesh Standard Time)
Fri Jan 01 1999 08:15:24 GMT+0600 (Bangladesh Standard Time)
Thu Jan 01 1998 08:15:24 GMT+0600 (Bangladesh Standard Time)
Wed Jan 01 1997 08:15:24 GMT+0600 (Bangladesh Standard Time)
Mon Jan 01 1996 08:15:24 GMT+0600 (Bangladesh Standard Time)
Sun Jan 01 1995 08:15:24 GMT+0600 (Bangladesh Standard Time)
Sat Jan 01 1994 08:15:24 GMT+0600 (Bangladesh Standard Time)
Fri Jan 01 1993 08:15:24 GMT+0600 (Bangladesh Standard Time)
Wed Jan 01 1992 08:15:24 GMT+0600 (Bangladesh Standard Time)
Tue Jan 01 1991 08:15:24 GMT+0600 (Bangladesh Standard Time)
Mon Jan 01 1990 08:15:24 GMT+0600 (Bangladesh Standard Time)
Sun Jan 01 1989 08:15:24 GMT+0600 (Bangladesh Standard Time)
Fri Jan 01 1988 08:15:24 GMT+0600 (Bangladesh Standard Time)
Thu Jan 01 1987 08:15:24 GMT+0600 (Bangladesh Standard Time)
Wed Jan 01 1986 08:15:24 GMT+0600 (Bangladesh Standard Time)
Tue Jan 01 1985 08:15:24 GMT+0600 (Bangladesh Standard Time)
Sun Jan 01 1984 08:15:24 GMT+0600 (Bangladesh Standard Time)
Sat Jan 01 1983 08:15:24 GMT+0600 (Bangladesh Standard Time)
Fri Jan 01 1982 08:15:24 GMT+0600 (Bangladesh Standard Time)
Thu Jan 01 1981 08:15:24 GMT+0600 (Bangladesh Standard Time)
Tue Jan 01 1980 08:15:24 GMT+0600 (Bangladesh Standard Time)
Mon Jan 01 1979 08:15:24 GMT+0600 (Bangladesh Standard Time)
Sun Jan 01 1978 08:15:24 GMT+0600 (Bangladesh Standard Time)
Sat Jan 01 1977 08:15:24 GMT+0600 (Bangladesh Standard Time)
Thu Jan 01 1976 08:15:24 GMT+0600 (Bangladesh Standard Time)
Wed Jan 01 1975 08:15:24 GMT+0600 (Bangladesh Standard Time)
Tue Jan 01 1974 08:15:24 GMT+0600 (Bangladesh Standard Time)
Mon Jan 01 1973 08:15:24 GMT+0600 (Bangladesh Standard Time)
Sat Jan 01 1972 08:15:24 GMT+0600 (Bangladesh Standard Time)
Fri Jan 01 1971 08:15:24 GMT+0600 (Bangladesh Standard Time)
Thu Jan 01 1970 08:15:24 GMT+0600 (Bangladesh Standard Time)
Wed Jan 01 1969 08:15:24 GMT+0600 (Bangladesh Standard Time)
Mon Jan 01 1968 08:15:24 GMT+0600 (Bangladesh Standard Time)
Sun Jan 01 1967 08:15:24 GMT+0600 (Bangladesh Standard Time)
Sat Jan 01 1966 08:15:24 GMT+0600 (Bangladesh Standard Time)
Fri Jan 01 1965 08:15:24 GMT+0600 (Bangladesh Standard Time)
Wed Jan 01 1964 08:15:24 GMT+0600 (Bangladesh Standard Time)
Tue Jan 01 1963 08:15:24 GMT+0600 (Bangladesh Standard Time)
Mon Jan 01 1962 08:15:24 GMT+0600 (Bangladesh Standard Time)
Sun Jan 01 1961 08:15:24 GMT+0600 (Bangladesh Standard Time)
Fri Jan 01 1960 08:15:24 GMT+0600 (Bangladesh Standard Time)
Thu Jan 01 1959 08:15:24 GMT+0600 (Bangladesh Standard Time)
Wed Jan 01 1958 08:15:24 GMT+0600 (Bangladesh Standard Time)
Tue Jan 01 1957 08:15:24 GMT+0600 (Bangladesh Standard Time)
Sun Jan 01 1956 08:15:24 GMT+0600 (Bangladesh Standard Time)
Sat Jan 01 1955 08:15:24 GMT+0600 (Bangladesh Standard Time)
Fri Jan 01 1954 08:15:24 GMT+0600 (Bangladesh Standard Time)
Thu Jan 01 1953 08:15:24 GMT+0600 (Bangladesh Standard Time)
Tue Jan 01 1952 08:15:24 GMT+0600 (Bangladesh Standard Time)
Mon Jan 01 1951 08:45:24 GMT+0630 (Bangladesh Standard Time)
Sun Jan 01 1950 08:45:24 GMT+0630 (Bangladesh Standard Time)
Sat Jan 01 1949 08:45:24 GMT+0630 (Bangladesh Standard Time)
Thu Jan 01 1948 08:45:24 GMT+0630 (Bangladesh Standard Time)
Wed Jan 01 1947 08:45:24 GMT+0630 (Bangladesh Standard Time)
Tue Jan 01 1946 08:45:24 GMT+0630 (Bangladesh Standard Time)
Mon Jan 01 1945 08:45:24 GMT+0630 (Bangladesh Standard Time)
Sat Jan 01 1944 08:45:24 GMT+0630 (Bangladesh Standard Time)
Fri Jan 01 1943 08:45:24 GMT+0630 (Bangladesh Standard Time)
Thu Jan 01 1942 08:45:24 GMT+0630 (Bangladesh Standard Time)
Wed Jan 01 1941 08:08:44 GMT+0553 (Bangladesh Standard Time)
Mon Jan 01 1940 08:08:44 GMT+0553 (Bangladesh Standard Time)
Sun Jan 01 1939 08:08:44 GMT+0553 (Bangladesh Standard Time)
Sat Jan 01 1938 08:08:44 GMT+0553 (Bangladesh Standard Time)
Fri Jan 01 1937 08:08:44 GMT+0553 (Bangladesh Standard Time)
Wed Jan 01 1936 08:08:44 GMT+0553 (Bangladesh Standard Time)
Tue Jan 01 1935 08:08:44 GMT+0553 (Bangladesh Standard Time)
Mon Jan 01 1934 08:08:44 GMT+0553 (Bangladesh Standard Time)
Sun Jan 01 1933 08:08:44 GMT+0553 (Bangladesh Standard Time)
Fri Jan 01 1932 08:08:44 GMT+0553 (Bangladesh Standard Time)
Thu Jan 01 1931 08:08:44 GMT+0553 (Bangladesh Standard Time)
Wed Jan 01 1930 08:08:44 GMT+0553 (Bangladesh Standard Time)
Tue Jan 01 1929 08:08:44 GMT+0553 (Bangladesh Standard Time)
Sun Jan 01 1928 08:08:44 GMT+0553 (Bangladesh Standard Time)
Sat Jan 01 1927 08:08:44 GMT+0553 (Bangladesh Standard Time)
Fri Jan 01 1926 08:08:44 GMT+0553 (Bangladesh Standard Time)
Thu Jan 01 1925 08:08:44 GMT+0553 (Bangladesh Standard Time)
Tue Jan 01 1924 08:08:44 GMT+0553 (Bangladesh Standard Time)
Mon Jan 01 1923 08:08:44 GMT+0553 (Bangladesh Standard Time)
Sun Jan 01 1922 08:08:44 GMT+0553 (Bangladesh Standard Time)
Sat Jan 01 1921 08:08:44 GMT+0553 (Bangladesh Standard Time)
Thu Jan 01 1920 08:08:44 GMT+0553 (Bangladesh Standard Time)
Wed Jan 01 1919 08:08:44 GMT+0553 (Bangladesh Standard Time)
Tue Jan 01 1918 08:08:44 GMT+0553 (Bangladesh Standard Time)
Mon Jan 01 1917 08:08:44 GMT+0553 (Bangladesh Standard Time)
Sat Jan 01 1916 08:08:44 GMT+0553 (Bangladesh Standard Time)
Fri Jan 01 1915 08:08:44 GMT+0553 (Bangladesh Standard Time)
Thu Jan 01 1914 08:08:44 GMT+0553 (Bangladesh Standard Time)
Wed Jan 01 1913 08:08:44 GMT+0553 (Bangladesh Standard Time)
Mon Jan 01 1912 08:08:44 GMT+0553 (Bangladesh Standard Time)
Sun Jan 01 1911 08:08:44 GMT+0553 (Bangladesh Standard Time)
Sat Jan 01 1910 08:08:44 GMT+0553 (Bangladesh Standard Time)
Fri Jan 01 1909 08:08:44 GMT+0553 (Bangladesh Standard Time)
Wed Jan 01 1908 08:08:44 GMT+0553 (Bangladesh Standard Time)
Tue Jan 01 1907 08:08:44 GMT+0553 (Bangladesh Standard Time)
Mon Jan 01 1906 08:08:44 GMT+0553 (Bangladesh Standard Time)
Sun Jan 01 1905 08:08:44 GMT+0553 (Bangladesh Standard Time)
Fri Jan 01 1904 08:08:44 GMT+0553 (Bangladesh Standard Time)
Thu Jan 01 1903 08:08:44 GMT+0553 (Bangladesh Standard Time)
Wed Jan 01 1902 08:08:44 GMT+0553 (Bangladesh Standard Time)
Tue Jan 01 1901 08:08:44 GMT+0553 (Bangladesh Standard Time)
Mon Jan 01 1900 08:08:44 GMT+0553 (Bangladesh Standard Time)
Sun Jan 01 1899 08:08:44 GMT+0553 (Bangladesh Standard Time)
Sat Jan 01 1898 08:08:44 GMT+0553 (Bangladesh Standard Time)
Fri Jan 01 1897 08:08:44 GMT+0553 (Bangladesh Standard Time)
Wed Jan 01 1896 08:08:44 GMT+0553 (Bangladesh Standard Time)
Tue Jan 01 1895 08:08:44 GMT+0553 (Bangladesh Standard Time)
Mon Jan 01 1894 08:08:44 GMT+0553 (Bangladesh Standard Time)
Sun Jan 01 1893 08:08:44 GMT+0553 (Bangladesh Standard Time)
Fri Jan 01 1892 08:08:44 GMT+0553 (Bangladesh Standard Time)
Thu Jan 01 1891 08:08:44 GMT+0553 (Bangladesh Standard Time)
Wed Jan 01 1890 08:08:44 GMT+0553 (Bangladesh Standard Time)
Tue Jan 01 1889 08:17:04 GMT+0601 (Bangladesh Standard Time)
Sun Jan 01 1888 08:17:04 GMT+0601 (Bangladesh Standard Time)
Sat Jan 01 1887 08:17:04 GMT+0601 (Bangladesh Standard Time)
Fri Jan 01 1886 08:17:04 GMT+0601 (Bangladesh Standard Time)
Thu Jan 01 1885 08:17:04 GMT+0601 (Bangladesh Standard Time)
Tue Jan 01 1884 08:17:04 GMT+0601 (Bangladesh Standard Time)
Mon Jan 01 1883 08:17:04 GMT+0601 (Bangladesh Standard Time)
Sun Jan 01 1882 08:17:04 GMT+0601 (Bangladesh Standard Time)
Sat Jan 01 1881 08:17:04 GMT+0601 (Bangladesh Standard Time)
Thu Jan 01 1880 08:17:04 GMT+0601 (Bangladesh Standard Time)
Wed Jan 01 1879 08:17:04 GMT+0601 (Bangladesh Standard Time)
Tue Jan 01 1878 08:17:04 GMT+0601 (Bangladesh Standard Time)
Mon Jan 01 1877 08:17:04 GMT+0601 (Bangladesh Standard Time)
Sat Jan 01 1876 08:17:04 GMT+0601 (Bangladesh Standard Time)
Fri Jan 01 1875 08:17:04 GMT+0601 (Bangladesh Standard Time)
Thu Jan 01 1874 08:17:04 GMT+0601 (Bangladesh Standard Time)
Wed Jan 01 1873 08:17:04 GMT+0601 (Bangladesh Standard Time)
Mon Jan 01 1872 08:17:04 GMT+0601 (Bangladesh Standard Time)
Sun Jan 01 1871 08:17:04 GMT+0601 (Bangladesh Standard Time)
Sat Jan 01 1870 08:17:04 GMT+0601 (Bangladesh Standard Time)
Fri Jan 01 1869 08:17:04 GMT+0601 (Bangladesh Standard Time)
Wed Jan 01 1868 08:17:04 GMT+0601 (Bangladesh Standard Time)
Tue Jan 01 1867 08:17:04 GMT+0601 (Bangladesh Standard Time)
Mon Jan 01 1866 08:17:04 GMT+0601 (Bangladesh Standard Time)
Sun Jan 01 1865 08:17:04 GMT+0601 (Bangladesh Standard Time)
Fri Jan 01 1864 08:17:04 GMT+0601 (Bangladesh Standard Time)
Thu Jan 01 1863 08:17:04 GMT+0601 (Bangladesh Standard Time)
Wed Jan 01 1862 08:17:04 GMT+0601 (Bangladesh Standard Time)
Tue Jan 01 1861 08:17:04 GMT+0601 (Bangladesh Standard Time)
Sun Jan 01 1860 08:17:04 GMT+0601 (Bangladesh Standard Time)
Sat Jan 01 1859 08:17:04 GMT+0601 (Bangladesh Standard Time)
Fri Jan 01 1858 08:17:04 GMT+0601 (Bangladesh Standard Time)
Thu Jan 01 1857 08:17:04 GMT+0601 (Bangladesh Standard Time)
Tue Jan 01 1856 08:17:04 GMT+0601 (Bangladesh Standard Time)
Mon Jan 01 1855 08:17:04 GMT+0601 (Bangladesh Standard Time)
Sun Jan 01 1854 08:17:04 GMT+0601 (Bangladesh Standard Time)
Sat Jan 01 1853 08:17:04 GMT+0601 (Bangladesh Standard Time)
Thu Jan 01 1852 08:17:04 GMT+0601 (Bangladesh Standard Time)
Wed Jan 01 1851 08:17:04 GMT+0601 (Bangladesh Standard Time)
Tue Jan 01 1850 08:17:04 GMT+0601 (Bangladesh Standard Time)
Mon Jan 01 1849 08:17:04 GMT+0601 (Bangladesh Standard Time)
Sat Jan 01 1848 08:17:04 GMT+0601 (Bangladesh Standard Time)
Fri Jan 01 1847 08:17:04 GMT+0601 (Bangladesh Standard Time)
Thu Jan 01 1846 08:17:04 GMT+0601 (Bangladesh Standard Time)
Wed Jan 01 1845 08:17:04 GMT+0601 (Bangladesh Standard Time)
Mon Jan 01 1844 08:17:04 GMT+0601 (Bangladesh Standard Time)
Sun Jan 01 1843 08:17:04 GMT+0601 (Bangladesh Standard Time)
Sat Jan 01 1842 08:17:04 GMT+0601 (Bangladesh Standard Time)
Fri Jan 01 1841 08:17:04 GMT+0601 (Bangladesh Standard Time)
Wed Jan 01 1840 08:17:04 GMT+0601 (Bangladesh Standard Time)
Tue Jan 01 1839 08:17:04 GMT+0601 (Bangladesh Standard Time)
Mon Jan 01 1838 08:17:04 GMT+0601 (Bangladesh Standard Time)
Sun Jan 01 1837 08:17:04 GMT+0601 (Bangladesh Standard Time)
Fri Jan 01 1836 08:17:04 GMT+0601 (Bangladesh Standard Time)
Thu Jan 01 1835 08:17:04 GMT+0601 (Bangladesh Standard Time)
Wed Jan 01 1834 08:17:04 GMT+0601 (Bangladesh Standard Time)
Tue Jan 01 1833 08:17:04 GMT+0601 (Bangladesh Standard Time)
Sun Jan 01 1832 08:17:04 GMT+0601 (Bangladesh Standard Time)
Sat Jan 01 1831 08:17:04 GMT+0601 (Bangladesh Standard Time)
Fri Jan 01 1830 08:17:04 GMT+0601 (Bangladesh Standard Time)
Thu Jan 01 1829 08:17:04 GMT+0601 (Bangladesh Standard Time)
Tue Jan 01 1828 08:17:04 GMT+0601 (Bangladesh Standard Time)
Mon Jan 01 1827 08:17:04 GMT+0601 (Bangladesh Standard Time)
Sun Jan 01 1826 08:17:04 GMT+0601 (Bangladesh Standard Time)
Sat Jan 01 1825 08:17:04 GMT+0601 (Bangladesh Standard Time)
Thu Jan 01 1824 08:17:04 GMT+0601 (Bangladesh Standard Time)
Wed Jan 01 1823 08:17:04 GMT+0601 (Bangladesh Standard Time)
Tue Jan 01 1822 08:17:04 GMT+0601 (Bangladesh Standard Time)
Mon Jan 01 1821 08:17:04 GMT+0601 (Bangladesh Standard Time)
Sat Jan 01 1820 08:17:04 GMT+0601 (Bangladesh Standard Time)
.....
.....

If this code is executed in chrome, it generates the same output. Also the output is same if I replace the line test.setUTCFullYear(i, 0, 1); in the code with test.setFullYear(i, 0, 1);.

As you can see in the output, up until the year 1952 - the time zone in the date shows GMT+0600 - but after that, the time zone is changed to GMT+0630 - then from the year 1941, the time zone is GMT+0553; and from the year 1889 onwards, the time zone is GMT+0601.

From the documentation of setFullYear():

The setFullYear() method of Date instances changes the year, month, and/or day of month for this date according to local time.

(According to the documentation of setUTCFullYear(), the same changes are made accroding to UTC time.)

My questions are:

The geographic location from where the date object is generated is same in all of the cases, still there are these changes in time zones (moreover - the time zones GMT+0553, GMT+0601 are not even present in the current world map). What is the reason behind such changes in time zones? What fact am I missing behind this behavior?
If we are to store a number of time values in the database, and then fetched those values to the front end; we have to use the Date object to represent the time values in javascript, as there is no "time only" object / variable type in javascript. How can the time only values from database be converted to Date object in javascript considering the changes in time zone, so that the time can be shown correctly in front end and the values can be used for different kind of calculation (i.e.- whether a time is earlier than another time, duration between the times, etc.) ?

ECMAScript *Date* objects respect historical offset and timezone changes. Checking at [*timeanddate.com*](https://www.timeanddate.com/time/zone/bangladesh/dhaka) shows that Bangladesh observed daylight saving in 2009, in 1971 the timezone name was changed, in 1951 the offset changed from +6:30 to +6:00, and so on. :-) — RobG, Aug 12 '23 at 10:48
Until about 1900, most places had an offset based on local mean noon, hence offsets like +5:53:20. Pretty much every major town had its own offset. However, faster long distance travel, initially by train and later aircraft, meant that scheduling connections and travel times could be confusing. Hence the adoption of fewer timezones based mostly on multiples of 1 hour offsets, with some 30 minute and the occasional 15 minute offset. — RobG, Aug 12 '23 at 10:51
Well, it is an understandable explanation. It would be better if this explanation was provided as an actual answer - while the referenced question has some related answers, it has no accepted answer; which is probably why I could not find out this information when I was searching for it. Anyway, thank you for providing the reply. — Muntasir, Aug 12 '23 at 12:59

Change of time zone after using setFullYear / setUTCFullYear

0 Answers0