How to convert std chrono time point to calendar datetime string with fractional seconds

Question

How to convert std  chrono  time point to calendar datetime string with fractional seconds   For example     10-10-2012 12 38 40 123456

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In general  you can t do this in any straightforward fashion  time point is essentially just a duration from a clock-specific epoch   If you have a std  chrono  system clock  time point  then you can use std  chrono  system clock  to time t to convert the time point to a time t  and then use the normal C functions such as ctime or strftime to format it     Example code   std  chrono  system clock  time point tp   std  chrono  system clock  now    std  time t time   std  chrono  system clock  to time t tp   std  tm timetm    std  localtime  amp time   std  cout  lt  lt   output      lt  lt  std  put time  amp timetm    c  Z    lt  lt                 lt  lt  std  chrono  duration cast lt std  chrono  milliseconds gt  tp time since epoch    count     1000  lt  lt  std  endl

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This worked for me for a format like YYYY MM DD-HH MM SS fff  Attempting to make this code capable of accepting any string format will be like reinventing the wheel  i e  there are functions for all this in Boost      std  chrono  system clock  time point string to time point const std  string  amp str        using namespace std      using namespace std  chrono       int yyyy  mm  dd  HH  MM  SS  fff       char scanf format       4d  2d  2d- 2d  2d  2d  3d        sscanf str c str    scanf format   amp yyyy   amp mm   amp dd   amp HH   amp MM   amp SS   amp fff        tm ttm   tm        ttm tm year   yyyy - 1900     Year since 1900     ttm tm mon   mm - 1     Month since January      ttm tm mday   dd     Day of the month  1-31      ttm tm hour   HH     Hour of the day  00-23      ttm tm min   MM      ttm tm sec   SS       time t ttime t   mktime  amp ttm        system clock  time point time point result   std  chrono  system clock  from time t ttime t        time point result    std  chrono  milliseconds fff       return time point result     std  string time point to string std  chrono  system clock  time point  amp tp        using namespace std      using namespace std  chrono       auto ttime t   system clock  to time t tp       auto tp sec   system clock  from time t ttime t       milliseconds ms   duration cast lt milliseconds gt  tp - tp sec        std  tm   ttm   localtime  amp ttime t        char date time format       Y  m  d- H  M  S        char time str      yyyy mm dd HH-MM SS fff        strftime time str  strlen time str   date time format  ttm        string result time str       result append           result append to string ms count           return result

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If you are to format a system clock  time point in the format of numpy datetime64  you could use   std  string format time point system clock  time point point        static assert system clock  time point  period  den    1000000000  amp  amp  system clock  time point  period  num    1       std  string out 29   0        char  buf    amp out 0       std  time t now c   system clock  to time t point       std  strftime buf  21    Y- m- dT H  M  S    std  localtime  amp now c        sprintf buf 20    09ld   point time since epoch   count     1000000000       return out      sample output  2019-11-19T17 59 58 425802666

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Self-explanatory code follows which first creates a std  tm corresponding to 10-10-2012 12 38 40  converts that to a std  chrono  system clock  time point  adds 0 123456 seconds  and then prints that out by converting back to a std  tm   How to handle the fractional seconds is in the very last step   include  lt iostream gt   include  lt chrono gt   include  lt ctime gt   int main            Create 10-10-2012 12 38 40 UTC as a std  tm     std  tm tm    0       tm tm sec   40      tm tm min   38      tm tm hour   12      tm tm mday   10      tm tm mon   9      tm tm year   112      tm tm isdst   -1         Convert std  tm to std  time t  popular extension      std  time t tt   timegm  amp tm          Convert std  time t to std  chrono  system clock  time point     std  chrono  system clock  time point tp                                         std  chrono  system clock  from time t tt          Add 0 123456 seconds        This will not compile if std  chrono  system clock  time point has          courser resolution than microseconds     tp    std  chrono  microseconds 123456               Now output tp         Convert std  chrono  system clock  time point to std  time t     tt   std  chrono  system clock  to time t tp          Convert std  time t to std  tm  popular extension      tm   std  tm 0       gmtime r  amp tt   amp tm          Output month     std  cout  lt  lt  tm tm mon   1  lt  lt   -          Output day     std  cout  lt  lt  tm tm mday  lt  lt   -          Output year     std  cout  lt  lt  tm tm year 1900  lt  lt              Output hour     if  tm tm hour  lt   9          std  cout  lt  lt   0       std  cout  lt  lt  tm tm hour  lt  lt              Output minute     if  tm tm min  lt   9          std  cout  lt  lt   0       std  cout  lt  lt  tm tm min  lt  lt              Output seconds with fraction          This is the heart of the question answer           First create a double-based second     std  chrono  duration lt double gt  sec   tp -                                      std  chrono  system clock  from time t tt                                        std  chrono  seconds tm tm sec            Then print out that double using whatever format you prefer      if  sec count    lt  10          std  cout  lt  lt   0       std  cout  lt  lt  std  fixed  lt  lt  sec count    lt  lt    n      For me this outputs  10-10-2012 12 38 40 123456  Your std  chrono  system clock  time point may or may not be precise enough to hold microseconds  Update An easier way is to just use this date library   The code simplifies down to  using C  14 duration literals    include  quot date h quot   include  lt iostream gt   include  lt type traits gt   int main         using namespace date      using namespace std  chrono      auto t   sys days 10 d 10 2012    12h   38min   40s   123456us      static assert std  is same lt decltype t                                  time point lt system clock  microseconds gt  gt      quot  quot        std  cout  lt  lt  t  lt  lt    n      which outputs  2012-10-10 12 38 40 123456  You can skip the static assert if you don t need to prove that the type of t is a std  chrono  time point  If the output isn t to your liking  for example you would really like dd-mm-yyyy ordering  you could   include  quot date h quot   include  lt iomanip gt   include  lt iostream gt   int main         using namespace date      using namespace std  chrono      using namespace std      auto t   sys days 10 d 10 2012    12h   38min   40s   123456us      auto dp   floor lt days gt  t       auto time   make time t-dp       auto ymd   year month day dp       cout fill  0        cout  lt  lt  ymd day    lt  lt   -   lt  lt  setw 2   lt  lt  static cast lt unsigned gt  ymd month              lt  lt   -   lt  lt  ymd year    lt  lt       lt  lt  time  lt  lt    n      which gives exactly the requested output  10-10-2012 12 38 40 123456  Update Here is how to neatly format the current time UTC with milliseconds precision   include  quot date h quot   include  lt iostream gt   int main         using namespace std  chrono      std  cout  lt  lt  date  format  quot  F  T n quot   time point cast lt milliseconds gt  system clock  now         which just output for me  2016-10-17 16 36 02 975  C  17 will allow you to replace time point cast lt milliseconds gt  with floor lt milliseconds gt    Until then date  floor is available in  quot date h quot   std  cout  lt  lt  date  format  quot  F  T n quot   date  floor lt milliseconds gt  system clock  now       Update C  20 In C  20 this is now simply   include  lt chrono gt   include  lt iostream gt   int main         using namespace std  chrono      auto t   sys days 10d 10 2012    12h   38min   40s   123456us      std  cout  lt  lt  t  lt  lt    n      Or just  std  cout  lt  lt  std  chrono  system clock  now    lt  lt    n    std  format will be available to customize the output

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In my case I use chrono and c function localtime r which is thread-safe  in opposition to std  localtime     include  lt iostream gt   include  lt chrono gt   include  lt ctime gt   include  lt time h gt   include  lt iomanip gt    int main       std  chrono  system clock  time point now   std  chrono  system clock  now      std  time t currentTime   std  chrono  system clock  to time t now     std  chrono  milliseconds now2   std  chrono  duration cast lt std  chrono  milliseconds gt  now time since epoch       struct tm currentLocalTime    localtime r  amp currentTime   amp currentLocalTime     char timeBuffer 80     std  size t charCount   std  strftime  timeBuffer  80                                             b  d  T                                              amp currentLocalTime                                 if  charCount    0  return -1     std  cout  lt  lt  timeBuffer  lt  lt       lt  lt  std  setfill  0    lt  lt  std  setw 3   lt  lt  now2 count     1000  lt  lt  std  endl    return 0

User · Answer

If system clock  this class have time t conversion    include  lt iostream gt   include  lt chrono gt   include  lt ctime gt   using namespace std  chrono   int main       system clock  time point p   system clock  now       std  time t t   system clock  to time t p     std  cout  lt  lt  std  ctime  amp t   lt  lt  std  endl     for example   Tue Sep 27 14 21 13 2011     example result   Thu Oct 11 19 10 24 2012   EDIT  But  time t does not contain fractional seconds  Alternative way is to use time point  time since epoch   function  This function returns duration from epoch  Follow example is milli second resolution s fractional    include  lt iostream gt   include  lt chrono gt   include  lt ctime gt   using namespace std  chrono   int main       high resolution clock  time point p   high resolution clock  now       milliseconds ms   duration cast lt milliseconds gt  p time since epoch        seconds s   duration cast lt seconds gt  ms     std  time t t   s count      std  size t fractional seconds   ms count     1000     std  cout  lt  lt  std  ctime  amp t   lt  lt  std  endl    std  cout  lt  lt  fractional seconds  lt  lt  std  endl      example result   Thu Oct 11 19 10 24 2012  925

User · Answer

I would have put this in a comment on the accepted answer  since that s where it belongs  but I can t   So  just in case anyone gets unreliable results  this could be why   Be careful of the accepted answer  it fails if the time point is before the epoch   This line of code   std  size t fractional seconds   ms count     1000    will yield unexpected values if ms count   is negative  since size t is not meant to hold negative values

[c++] How to convert std::chrono::time_point to calendar datetime string with fractional seconds?

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