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Say I have a specific instant in time where I know the hour, minute, day, second, month, year, etc; how can I convert this epoch time (seconds since 1970)?
I can't use Boost, so please don't suggest a Boost solution.
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For example, you could convert the time to an epoch and subtract it from another epoch value to quickly determine the difference. If the difference was 176,400 and you used the above chart or a math formula to find the difference is 2 days and 1 hour (86400 + 86400 + 3600 = 176,400).
In a computing context, an epoch is the date and time relative to which a computer's clock and timestamp values are determined. The epoch traditionally corresponds to 0 hours, 0 minutes, and 0 seconds (00:00:00) Coordinated Universal Time (UTC) on a specific date, which varies from system to system.
Select a blank cell, suppose Cell C2, and type this formula =(C2-DATE(1970,1,1))*86400 into it and press Enter key, if you need, you can apply a range with this formula by dragging the autofill handle. Now a range of date cells have been converted to Unix timestamps.
Use the mktime(3) function. For example:
struct tm t = {0}; // Initalize to all 0's
t.tm_year = 112; // This is year-1900, so 112 = 2012
t.tm_mon = 8;
t.tm_mday = 15;
t.tm_hour = 21;
t.tm_min = 54;
t.tm_sec = 13;
time_t timeSinceEpoch = mktime(&t);
// Result: 1347764053
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