Convert between boost dates and Unix timestamps (time_t)

date_conversion.cpp

#include <ctime>
#include <iostream>

#include <boost/date_time/gregorian/gregorian.hpp>
#include <boost/date_time/posix_time/posix_time.hpp>


//==============================================================================
//! Convert date part of Unix timestamp (time_t) to boost date
//!
//! time_t contains the number of seconds since 00:00 hours, Jan 1, 1970 UTC
//! 
//! \param time unix timestamp to convert
//! \return boost date of the timestamp's date
//
boost::gregorian::date to_bdate(time_t time) {
	return boost::posix_time::from_time_t(time).date();
}

//==============================================================================
//! Convert boost::gregorian::date to Unix timestamp (time_t)
//!
//! Converts a boost date to a Unix timestamp pointing to 00:00 hours, 0 seconds
//! of the day given by date.
//!
//! time_t contains the number of seconds since 00:00 hours, Jan 1, 1970 UTC.
//!
//! \param date to convert
//! \return unix timestamp 
time_t to_time_t(const boost::gregorian::date& date ){
	using namespace boost::posix_time;
	static ptime epoch(boost::gregorian::date(1970, 1, 1));
	time_duration::sec_type secs = (ptime(date,seconds(0)) - epoch).total_seconds();
	return time_t(secs);
}



using namespace std;
namespace bd = boost::gregorian;
namespace pt = boost::posix_time;


int main() {

	// get current time
	time_t nowtime;
	time(&nowtime);
	cout << "Unix timestamp of now:          " << nowtime << endl;

	// conversion time_t -> boost date
	bd::date today(to_bdate(nowtime));
	cout << "Timestamp converted to date:   " << today << endl;

	// conversion boost date -> time_t
	time_t thismorning = to_time_t(today);
	cout << "Date converted back to time_t:  " << thismorning << endl;

}

total_seconds() returns long (32 bits even on 64 bits platform compiler), so, you'll get integer overflow

In the latest boost total_seconds() ends up as an int64_t. See posix_time_system.hpp.