{"id":2763,"date":"2013-11-01T07:00:00","date_gmt":"2013-11-01T07:00:00","guid":{"rendered":"https:\/\/blogs.msdn.microsoft.com\/oldnewthing\/2013\/11\/01\/on-the-various-ways-of-getting-the-current-time-and-date-in-win32\/"},"modified":"2013-11-01T07:00:00","modified_gmt":"2013-11-01T07:00:00","slug":"on-the-various-ways-of-getting-the-current-time-and-date-in-win32","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20131101-00\/?p=2763","title":{"rendered":"On the various ways of getting the current time and date in Win32"},"content":{"rendered":"

\nThere are a number of functions in Win32\nthat obtain the current date and time.\nHere’s how they fit together:\n<\/p>\n

\nThe starting point is\nGet­System­Time­As­File­Time<\/code>.\nThis returns the current time in UTC in the form of a\nFILE­TIME<\/code> structure.\nThis also happens to be the time format used internally by the system,\nso this value can be retrieved with a minimum of fuss.\n<\/p>\n

\nYou can also call\nGet­System­Time<\/code> which returns the current UTC time\nin the form of a\nSYSTEM­TIME<\/code> structure.\nTo do this, the operating system takes the\ncurrent FILE­TIME<\/code> and then calls\nthe moral equivalent of\nFile­Time­To­System­Time<\/code>,\nwhich\ndoes a boatload of gnarly math\nto decompose the\nFILE­TIME<\/code>\ninto year, month, day, hour, minute, second, and millisecond.\n<\/p>\n

\nMeanwhile, you can also get the current local time by taking\nthe\nFILE­TIME<\/code> returned by\nGet­System­Time­As­File­Time<\/code>,\nthen passing it to\nFile­Time­To­Local­File­Time<\/code>.\n<\/p>\n

\nAnd finally, there’s\nGet­Local­Time<\/code>,\nwhich does the same thing\nas\nGet­System­Time<\/code>,\nbut it starts with the local file time.\n<\/p>\n

\nIn equations:\n<\/p>\n\n\n\n\n\n\n
Format<\/th>\nTime zone<\/th>\nFunction<\/th>\nAlgorithm<\/th>\n<\/tr>\n
FILE­TIME<\/td>\nUTC<\/td>\nGet­System­Time­As­File­Time<\/td>\n(Native format)<\/td>\n<\/tr>\n
FILE­TIME<\/td>\nLocal<\/td>\n(None)<\/td>\nGet­System­Time­As­File­Time\n + File­Time­To­Local­File­Time<\/td>\n<\/tr>\n
SYSTEM­TIME<\/td>\nUTC<\/td>\nGet­System­Time <\/td>\nGet­System­Time­As­File­Time<\/td>\n+ File­Time­To­System­Time<\/span><\/td>\n<\/tr>\n
SYSTEM­TIME<\/td>\nLocal<\/td>\nGet­Local­Time <\/td>\nGet­System­Time­As­File­Time\n + File­Time­To­Local­File­Time<\/td>\n+ File­Time­To­System­Time<\/span><\/td>\n<\/tr>\n<\/table>\n

\nI happen to be a fan of commutative diagrams.\n(Though since there are no closed loops, there is nothing to commute.)\n<\/p>\n

\n
\n A 2-by-2 grid of boxes.\n The top row is labeled FILE­TIME; the bottom row is labeled\n SYSTEM­TIME.\n The first column is labeled UTC; the second column is labeled Local.\n The upper left box\n is labeled Get­System­Time­As­File­Time.\n There is an outgoing arrow to the right\n labeled\n File­Time­To­Local­File­Time leading to the box in the\n second column labeled None.\n There is an outgoing arrow downward labeled\n File­Time­To­System­Time\n leading to the box in the second row, first column,\n labeled Get­System­Time.\n From the box in the upper right corner labeled None,\n there is an outgoing arrow\n downward labeled\n File­Time­To­System­Time\n leading to the box in the second row, second column,\n labeled Get­Local­Time.\n<\/div>\n
UTC<\/div>\n
Local<\/div>\n
File­Time<\/div>\n
\n Get­System­Time­As­File­Time<\/div>\n
\n File­Time­To­Local­File­Time<\/div>\n
(None)<\/div>\n
\n File­Time­To­System­Time<\/div>\n
\n File­Time­To­System­Time<\/div>\n
SYSTEM­TIME<\/div>\n
\n Get­System­Time<\/div>\n
\n Get­Local­Time<\/div>\n
\n
→<\/div>\n
↓<\/div>\n
↓<\/div>\n<\/p><\/div>\n<\/div>\n

\nTo complete the commutative diagram,\nthere would be an arrow connecting the bottom two boxes called\nSystem­Time­To­Local­Time<\/code>,\nbut there is no such function.\n<\/p>\n

\nToday’s article was inspired by some code I ran across which did this:\n<\/p>\n

\nSYSTEMTIME stNow;\nFILETIME ftNow;\nGetSystemTime(&stNow);\nSystemTimeToFileTime(&stNow, &ftNow);\n<\/pre>\n
\nThat code unwittingly takes an excursion from\nGet­System­Time­As­File­Time<\/code>\nthrough\nFile­Time­To­System­Time<\/code> to\nGet­System­Time<\/code>,\nthen back through\nSystem­Time­To­File­Time&shy<\/code> to\nreturn to\nGet­System­Time­As­File­Time<\/code>,\njust so that it can end up where it started,\nbut with a lot of extra math (and loss of resolution).\n<\/p>\n
\nExercise<\/b>: How would you implement the\nSystem­Time­To­Local­Time<\/code>\nfunction?<\/p>\n","protected":false},"excerpt":{"rendered":"
There are a number of functions in Win32 that obtain the current date and time. Here’s how they fit together: The starting point is Get­System­Time­As­File­Time. This returns the current time in UTC in the form of a FILE­TIME structure. This also happens to be the time format used internally by the system, so this value […]<\/p>\n","protected":false},"author":1069,"featured_media":111744,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[1],"tags":[25,108],"class_list":["post-2763","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code","tag-time"],"acf":[],"blog_post_summary":"
There are a number of functions in Win32 that obtain the current date and time. Here’s how they fit together: The starting point is Get­System­Time­As­File­Time. This returns the current time in UTC in the form of a FILE­TIME structure. This also happens to be the time format used internally by the system, so this value […]<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/2763","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/users\/1069"}],"replies":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/comments?post=2763"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/2763\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media\/111744"}],"wp:attachment":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media?parent=2763"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=2763"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=2763"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}