I have a file with tab separated entries like this: 1230735660 12/31/2008 10:01:00 8.94 277880 1230735729 12/31/2008 10:02:09 8.95 279693 1230735791 12/31/2008 10:03:11 8.95 284743 etc. I don't assume which column represents the date so I read one line at the time and test each token if it's a date in the format "%m/%d/%Y %H:%M:%S". However, when I process the first token (1230735660) date_time thinks it's a date in my format and returns 5660-Dec-07 00:00:00 How come? It seems to think that 3 is a separator. Could anyone please shed some light? Thanks! Here is a short program that exemplifies the problem: // g++ -g -Wall -o dateIO -I/usr/local/include/boost-1_38 -L/usr/local/ lib/ dateIOTest.C -lboost_date_time-gcc43-mt-1_38 #include <iostream> #include <sstream> #include <cstdlib> #include <string> #include <boost/date_time/posix_time/posix_time.hpp> using namespace std; using namespace boost::posix_time; int main(int argc, char * argv[]) { ptime pt; string datestr("1230735660"); istringstream INSTR(datestr); time_input_facet * tifacet=new time_input_facet("%m/%d/%Y %H:%M:%S"); locale loc(INSTR.getloc(),tifacet); INSTR.imbue(loc); INSTR >> pt; cout << pt << endl; return 0; }