Gmtsub() function, of which about 70% is mutex lock/unlock to lazily It appears that almost all of the time is attributable to the
Linux time converter code#
I looked at a profile and cross-referenced the Darwin code in The primary test case forīM_YMDToUnix_DaysFromCivil 5 ns 5 ns 147238851 Return correct results around leap seconds. When we use the hms_to_time() function above, these algorithms Given here could be a drop-in replacement for timegm(). Interesting exercise to add these features so that the algorithms Not a completely apples-to-apples comparison. I did not need these features for my use case, so for me these The algorithms in this article do not calculate day of week or day of (day of week) values that it calculated from the day, month, and year. Input struct tm, including tm_yday (day of year) and tm_wday Similarly timegm() will write the normalized values back to the Of the month value must happen prior to calling the function. Of range, or can even crash with out-of-bounds array access if you are TheĪlgorithms given here can return incorrect results if the month is out Timegm(), that indicates December of the previous year. For example, if you pass a month of -1 to Unlike the algorithms in this article, the timegm() function The year -4800 is frequently chosen as a base out ofĬonvention, as it is before the Julian Day epoch, which is before any
I think any of theseĪlgorithms could likely be adjusted to support whatever window of Into the past (assuming 32-bit integers). However theĭays_from_civil() algorithm is coded to support over 10 million years Most of the algorithms given here use the year -4800 as their base,Īnd will return incorrect results for years before that. Them support to at least the date 100 (year 1 Million). That said, theseĪlgorithms effectively support much wider bounds than this. To be far beyond what is needed in practice. The interval is a very wide range of years, and likely On macOS, as its timegm() function fails for all years prior to Other, and that they match the results of timegm(). I verified all the algorithms in this article across a wide range ofĭates (- to 2, or Julian Date 1 to 10,000,000).įor every day in this range I checked that the outputs match each In fact, all the algorithms given in thisĪrticle compile to branchless code (on x86-64 using recent versions ofīefore we compare these algorithms for size and speed, a few notesĪbout their semantics and correctness. Note that the although this algorithm has some ternary operators, it * epoch_days(1970, 1, 1) = 0 */ int epoch_days_table ( int year, int month, int day ) This uses the Unix epoch instead of a Julian Day. Places, I just condensed them into the smallest and simplest algorithm Than the Fortran algorithm above, I wrote the following algorithm forĪll of the general techniques used here are published in various In the interest of having an algorithm that is easier to understand Improving Readability With A Lookup Table (the algorithm ensures that all numbers being divided are positive, as Using the int data type, which also rounds down for positive numbers It is trivial to convert this Fortran function to C It does not use any lookup table,Īnd yet it somehow encodes the irregular pattern of month lengths and
Linux time converter full#
Multiply this by 86,400 seconds and add hms_to_time() to get a full
Once we have mapped a Y/M/D to a total number of days, it is easy to The variables I, J, and K are the year, month, This epoch is very longĪgo, in 4713 BC, but it’s a simple matter to convert it to the UnixĮpoch of by subtracting 2440588, the Julian Day number of That have passed since the Julian Date Epoch.
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