Where is the extra 75 seconds coming from?

Question

While writing some unit tests on a Julian Day calculator, I found that dates prior to 2nd December 1847 were being initialised incorrectly by NSDate. They appear to have 75 seconds added on. I haven't been able to find anything pointing to that date (which is well after the Gregorian calendar cutoff). Is it a bug or is there a historic calendar adjustment that I've not come across?

int main(int argc, const char * argv[]) {     @autoreleasepool {          NSCalendar *cal = [NSCalendar currentCalendar];         NSDateComponents *dateComps = [NSDateComponents new];         dateComps.year = 1847;         dateComps.month = 12;         dateComps.day    = 1;         NSDate *d1 = [cal dateFromComponents:dateComps];         NSLog(@"d1 = %@", d1);          dateComps = [NSDateComponents new];         dateComps.year = 1847;         dateComps.month = 12;         dateComps.day    = 2;         NSDate *d2 = [cal dateFromComponents:dateComps];         NSLog(@"d2 = %@", d2);     }     return 0; } 

Output:

d1 = 1847-12-01 00:01:15 +0000

d2 = 1847-12-02 00:00:00 +0000

Answer 1

According to http://www.timeanddate.com/worldclock/clockchange.html?n=136&year=1847 there was a time shift forward of 75 seconds at that time.

In London, when local time was about to reach 12:00:00 AM on Wednesday December 1, 1847, clocks were advanced to Wednesday, December 1, 1847 12:01:15 AM.

Answer 2

In response to the post by Richard Krajunus, here is some additional information from the zoneinfo database used by most computers to track these kinds of changes:

# From Paul Eggert (1993-11-18): # # Howse writes that Britain was the first country to use standard time. # The railways cared most about the inconsistencies of local mean time, # and it was they who forced a uniform time on the country. # The original idea was credited to Dr. William Hyde Wollaston (1766-1828) # and was popularized by Abraham Follett Osler (1808-1903). # The first railway to adopt London time was the Great Western Railway # in November 1840; other railways followed suit, and by 1847 most # (though not all) railways used London time.  On 1847-09-22 the # Railway Clearing House, an industry standards body, recommended that GMT be # adopted at all stations as soon as the General Post Office permitted it. # The transition occurred on 12-01 for the L&NW, the Caledonian, # and presumably other railways; the January 1848 Bradshaw's lists many # railways as using GMT.  By 1855 the vast majority of public # clocks in Britain were set to GMT (though some, like the great clock # on Tom Tower at Christ Church, Oxford, were fitted with two minute hands, # one for local time and one for GMT).  The last major holdout was the legal # system, which stubbornly stuck to local time for many years, leading # to oddities like polls opening at 08:13 and closing at 16:13. # The legal system finally switched to GMT when the Statutes (Definition # of Time) Act took effect; it received the Royal Assent on 1880-08-02. # # In the tables below, we condense this complicated story into a single # transition date for London, namely 1847-12-01.  We don't know as much # about Dublin, so we use 1880-08-02, the legal transition time. 

Sorry I couldn't respond using a comment in that thread; StackOverflow does not deem me worthy of that yet.