int is long, select column can be function, some fixes..

just to get it work.. needs improvement

ml_date.cpp

@@ -0,0 +1,59 @@
+
+#include "ml_date.h"
+
+long now() {
+	// get-universal-time
+	time_t t = std::time(0);
+	long int now = static_cast<long int>(t);
+
+	return now;
+}
+
+std::string date_to_string(const long datetime, const std::string format) {
+	// std::locale::global(std::locale("en-US.UTF8"));
+
+	time_t timestamp = datetime;
+	char mbstr[128];
+
+	if (std::strftime(mbstr, sizeof(mbstr), format.c_str(), std::localtime(&timestamp))) {
+		std::string result = {mbstr};
+		return result;
+	}
+	// TODO exception here
+    return "invalid argument";
+}
+
+long string_to_date(const std::string &datestr, const std::string &format) {
+	// format for example "%d.%m.%Y";
+
+	std::istringstream in{datestr.c_str()};
+	date::sys_seconds tp;
+	in >> date::parse(format, tp);
+	return tp.time_since_epoch().count();
+}
+
+long add_to_date(const long datetime, const long quantity, const std::string &part) {
+	// part is one of 'year', 'month', 'day', 'hour', 'minute' or 'second'
+
+	// very basic implementation, just for now - no timezones DST etc
+	time_t base = datetime;
+	struct tm *tm = localtime(&base);
+
+	if (part == "year") {
+		tm->tm_year += quantity;
+	} else if (part == "month") {
+		tm->tm_mon += quantity;
+	} else if (part == "day") {
+		tm->tm_mday += quantity;
+	} else if (part == "hour") {
+		tm->tm_hour += quantity;
+	} else if (part == "minute") {
+		tm->tm_min += quantity;
+	} else if (part == "second") {
+		tm->tm_sec += quantity;
+	} else {
+		// TODO exception here
+	}
+
+	return mktime(tm);
+}