dosdate.cpp

#include <cstdint>
#include <cstdio>
#include <ctime>
#include <list>

using namespace std;

/*
MS-DOS date and MS-DOS time are data formats associated with MS-DOS.
They are used in some file formats from the MS-DOS era. Each is a
16-bit integer.

MS-DOS date represents a day in the range 1980 to 2099 (or maybe 2107,
but dates after 2099 aren't always correctly handled).

MS-DOS time represents a 2-second interval within some day. The time
is usually expected to be in "local time", and there is no indication
of the time zone. This makes it fairly useless in the internet age.

http://fileformats.archiveteam.org/wiki/MS-DOS_date/time
*/

tm dos_to_local_datetime(const uint16_t sec_count, const uint16_t day_count) {
	tm tim = { 0 };
	tim.tm_sec  = (sec_count & 0x1f) * 2;
	tim.tm_min  = (sec_count >> 5) & 0x3f;
	tim.tm_hour = (sec_count >> 11) & 0x1f;
	tim.tm_mday = day_count & 0x1f;
	tim.tm_mon  = ((day_count >> 5) & 0x0f) - 1;
	tim.tm_year = (day_count >> 9) + 1980 - 1900;
	tim.tm_isdst = -1;
	// serialize time
	mktime(&tim);
	return tim;
}

const char * day_to_str(uint16_t d)
{
	switch (d) {
	case 0:     return "1980-01-01";
	case 1:     return "1980-01-02";
	case 365:   return "1980-12-31";
	case 3653:  return "1990-01-01";
	case 7305:  return "2000-01-01";
	case 14694: return "2020-03-25";
	case 43465: return "2099-01-01";
	case 46386: return "2107-01-01";
	case 65535: return "2159-06-06";
	default:    return "???";
	}
}

int main()
{
	std::list<uint16_t> seconds = { 0, 1, 29, 30, 31, 21599, 21600, 21601, 43199, 43200, 43201, 65535 }; 
	std::list<uint16_t> days = { 0, 1, 365, 3653, 7305, 14694, 43465, 46386, 65535 }; 
	char time_desc[100] = {};
	for (const auto second : seconds) {
		for (const auto day : days) {
			const tm tim = dos_to_local_datetime(second, day);
			strftime(time_desc, sizeof(time_desc), "%F", &tim);
			printf("%6d, %6d | %10s | %10s\n",
			       second, day,
			       day_to_str(day),
			       time_desc);
		}
	}
	return 0;
}

gistfile1.txt

#include <cstdint>
#include <cstdio>
#include <ctime>
#include <list>

using namespace std;

/*
MS-DOS date and MS-DOS time are data formats associated with MS-DOS.
They are used in some file formats from the MS-DOS era. Each is a
16-bit integer.

MS-DOS date represents a day in the range 1980 to 2099 (or maybe 2107,
but dates after 2099 aren't always correctly handled).

MS-DOS time represents a 2-second interval within some day. The time
is usually expected to be in "local time", and there is no indication
of the time zone. This makes it fairly useless in the internet age.

http://fileformats.archiveteam.org/wiki/MS-DOS_date/time
*/

tm dos_to_local_datetime(const uint16_t sec_count, const uint16_t day_count) {
	tm tim = { 0 };
	tim.tm_sec  = (sec_count & 0x1f) * 2;
	tim.tm_min  = (sec_count >> 5) & 0x3f;
	tim.tm_hour = (sec_count >> 11) & 0x1f;
	tim.tm_mday = day_count & 0x1f;
	tim.tm_mon  = ((day_count >> 5) & 0x0f) - 1;
	tim.tm_year = (day_count >> 9) + 1980 - 1900;
	tim.tm_isdst = -1;
	// serialize time
	mktime(&tim);
	return tim;
}

const char * day_to_str(uint16_t d)
{
	switch (d) {
	case 0:     return "1980-01-01";
	case 1:     return "1980-01-02";
	case 365:   return "1980-12-31";
	case 3653:  return "1990-01-01";
	case 7305:  return "2000-01-01";
	case 14694: return "2020-03-25";
	case 43465: return "2099-01-01";
	case 46386: return "2107-01-01";
	case 65535: return "2159-06-06";
	default:    return "???";
	}
}

int main()
{
	std::list<uint16_t> seconds = { 0, 1, 29, 30, 31, 21599, 21600, 21601, 43199, 43200, 43201, 65535 }; 
	std::list<uint16_t> days = { 0, 1, 365, 3653, 7305, 14694, 43465, 46386, 65535 }; 
	char time_desc[100] = {};
	for (const auto second : seconds) {
		for (const auto day : days) {
			const tm tim = dos_to_local_datetime(second, day);
			strftime(time_desc, sizeof(time_desc), "%F", &tim);
			printf("%6d, %6d | %10s | %10s\n",
			       second, day,
			       day_to_str(day),
			       time_desc);
		}
	}
	return 0;
}