Finding difference between two dates using mktime()

Question

I am trying to find the difference between two dates in seconds.

I have used tm1 and tm2 to store the two different dates.

tm1 = 1900-01-01 00:00:00
tm2 = 2000-01-01 20:38:40

 

tm1.tm_year = 0  
tm1.tm_mon  = 0  
tm1.tm_mday = 1 

 

tm2.tm_year = 100  
tm2.tm_mon  = 0  
tm2.tm_mday = 1  
tm2.tm_hour = 20  
tm2.tm_min  = 38  
tm2.tm_sec  = 40  

Now to the code :

time_t t1, t2;
t1=mktime(&tm1);
t2=mktime(&tm2);
int diff = difftime(t2,t1);

The actual value of t1 in this case is 2209010000 and that of t2 is 946739320.

Now I executed a similar function using PostgreSQL :

select EXTRACT(EPOCH FROM ('2000-01-01 20:38:40' - CAST('1900-01-01' AS TIMESTAMP)));

But the result from difftime() function is 3155749320 and the result through querying is 3155747920, a difference of 1400 seconds.

Why is there a difference in the result obtained from both the methods? What would have caused the difference? Do I need to handle time zones?

Answer 1

The unix timestamp for a given date begins at 1/1/1970, so the first thing that hits me when you show your timestamps is that 1/1/1900 should be negative. If we analize the exact UNIX timestamp for the date you used, it should be -2208988800 (for 1/1/1900 00:00:00 UTC) which is larger (in absolute value) than the lowest minimum integer representable in 32 bits (in two's complement) which is -2147483648.

If you want to do exact calculations, you have to switch to 64bit integers, and you'll get the correct result.

• Unix timestamp for 2000-01-01 20:38:40 -> 946759120
• Unix timestamp for 1900-01-01 00:00:00 -> -2208988800
• difference between the two: 946759120 - (-2208988800) = 3155747920 s.

The timestamp you obtained are the 32 least signifiant bits of the overflown result. The minimum date possible with 32 bit timestamps in two's complement is for -2147483648 -> 13/Dec/1901, 20:45:52.