A datetime arithmetic in MQL4 - datetime

Question

I would like to define a datetime type variable that is a result of a simple arithmetic operation between datetime type variables.
I've defined:
datetime duration = ( TimeCurrent() - OrderOpenTime() );
datetime TmStop = StringToTime( "1970.01.01 16:00" );
but when I call it in some other arithmetic operation or generally in code like this
ExitBuy_H1 = ( duration > TmClose && ...
or this
text[3]= "Duration: " + TimeToStr( duration, TIME_MINUTES );
it doesn't work.
TmStop instead works fine.
Does anyone know why?

Answer 1

datetime is a simple integer, number of seconds passed since 1970.01.01 00:00. duration in your example is also in seconds, even though it is datetime formated, when you need it in minutes, divide by 60. TmClose from your example means 16*60*60 seconds and you can compare that integer with any other int of course, but what might be the reason for that?
if you hold you position more then 16 hours, then duration > TmClose is true. if you want to convert difference in seconds (duration) into time, then you will have time converted from 1970.01.01 00:00 + duration seconds.
Anyway it is not clear what is your goal in doing this calculations? if you want to make sure that you hold that particular position more then x hours, then simple bool holdMoreThanXHours = TimeCurrent()-OrderOpenTime()>x*PeriodSeconds(PERIOD_H1), and do not forget to reselect each ticket if you have several ones in open

Answer 2

Fact A) the code, as-is, absolutely out of any question works.
//+------------------------------------------------------------------+
//| Test_StackOverflow.mq4 |
//+------------------------------------------------------------------+
#property strict
void OnStart() {
datetime duration = ( TimeCurrent() - OrderOpenTime() );
string txt = "Duration: " + TimeToStr( duration, TIME_MINUTES );
}
//+------------------------------------------------------------------+
0 error(s), 0 warning(s), compile time: 2000 msec 1 1
Fact B) the full MCVE-context of the code, as-is, is missing.
StackOverflow requires users to post a complete MCVE-representation of the problem. This requirement was not met in the original post.
While the datetime and int data-types are mutually interchangeable, the problem does not seem to be hidden in this intrinsic "duality" of a value representation, but must be somewhere else.
The main suspects for Why? are:
variable definition was masked by another variable having the same name
variable scope-of-definition was exceeded ( asking a variable outside of it's scope )
db.Pool-operations were not preceded by OrderSelect()