Convert between date/time and time-stamp without using standard library routines

I am trying to implement two simple convertors: date/time to time-stamp and vice-versa, without any dependencies on time library routines (such as localtime, mktime, etc, mainly due to the fact that some of them are not thread-safe).

I have the following date/time structure:

typedef struct
{
    unsigned char second; // 0-59
    unsigned char minute; // 0-59
    unsigned char hour;   // 0-23
    unsigned char day;    // 1-31
    unsigned char month;  // 1-12
    unsigned char year;   // 0-99 (representing 2000-2099)
}
date_time_t;

I would like to have a second opinion on the following conversion routines (given a legal input):

```
static unsigned short days[4][12] =
{
{ 0, 31, 60, 91, 121, 152, 182, 213, 244, 274, 305, 335},
{ 366, 397, 425, 456, 486, 517, 547, 578, 609, 639, 670, 700},
{ 731, 762, 790, 821, 851, 882, 912, 943, 974,1004,1035,1065},
{1096,1127,1155,1186,1216,1247,1277,1308,1339,1369,1400,1430},
};

unsigned int date_time_to_epoch(date_time_t* date_time)
{
unsigned int second = date_time->second; // 0-59
unsigned int minute = date_time->minute; // 0-59
unsigned int hour = date_time->hour; // 0-23
unsigned int day = date_time->day-1; // 0-30
unsigned int month = date_time->month-1; // 0-11
unsigned int year = date_time->year; // 0-99
return (((year/4(3654+1)+days[year%4][month]+day)24+hour)60+minute)*60+second;
}

void epoch_to_date_time(date_time_t* date_time,unsigned int epoch)
{
date_time->second = epoch%60; epoch /= 60;
date_time->minute = epoch%60; epoch /= 60;
date_time->hour = epoch%24; epoch /= 24;

unsigned int years = epoch/(3654+1)4; epoch %= 365*4+1;

unsigned int year;
for (year=3; year>0; year--)
{
if (epoch >= days[year][0])
break;
}

unsigned int month;
for (month=11; month>0; month--)
{
if (epoch >= days[year][month])
break;
}

date_time->year = years+yea

-
Good that OP is using 4 simplifications: year 2000-2099, no DST, no leap second, no timezone. So OP knows of code limitations concerning these. Various elements of this function break without those givens.

-
Make static unsigned short days a const.

-
Use a long for your epoch as in:

void epoch_to_date_time(date_time_t* date_time,unsigned long epoch)

as unsigned is only guaranteed to range form 0 to at least 65535 which is insufficient here.

-
For various functions, consider adding const. There are pros and cons to this, but may be beneficial in your case:

// unsigned int date_time_to_epoch(date_time_t* date_time)
unsigned int date_time_to_epoch(const date_time_t* date_time)

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There exist equation based (non-loop) solutions to the year-day that do not need a table like your unsigned short days[4][12]. Code then looks more complicated, but is faster. Please advise if interested.

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Most of the magic numbers like 60, 24 are so well-known that constant or macro substitution seems superfluous. But (365*4+1) may benefit with something like "DaysPer4Years".

StackExchange Code Review Q#38275, answer score: 13