Thanks much for such quick responses! Now bear with me as I try to interpret with my nooby brain.
First, I see my obvious line 21 and floor mistake.
Next, half of what was said had no been learned yet (adding .0f or unsigned), but I did some research to figure out what those mean.
But  trying to avoid possibly using more advanced techniques as the book states to use float (unsigned, even though it is far less messy) and using what I have, I've been trying to figure out how to use this answer:
Looks like you have integer division, which will discard the fractional part.
e.g. startHr = s/100;
both s and 100 are integers.
You could use s/100.0f to specify that 100 is of type float, not int.
Here is my modification with error:
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#include <cstdlib>
#include <iostream>
#include <cmath>
using namespace std;
int calculations(int, int);
int main()
{
int start,
end;
cout << "Please input your start time in military time(example: 8:30=2030): ";
cin >> start;
cout << "Please input your end time: ";
cin >> end;
cout << "Your elapsed time is " << calculations(start, end) << " minutes.";
return 0;
}
int calculations(int s, int e) //calculations to determine the difference in time
{
float startHr, //separation of hours and minutes
endHr,
startDec, //hours and minutes in decimal form
endDec,
startMin, //minutes when separated from hours
endMin;
startHr = s/100.0f; //calculations to separate the hour to integer form
startHr=floor(startHr);
startMin = s%100.0f; //calculations to find remainder in minutes
startDec = startMin/60.0f + startHr; //converting minutes to decimal form and adding to hours
endHr = e/100.0f; //same calculations for end time
endHr=floor(endHr);
endMin = e%100.0f;
endDec = endMin/60.0f + endHr;
return (endDec  startDec)*60.0f; //return of final calculations in decimal form multiplied by 60 to convert to minutes
}
 
I receive an error:
error: invalid operands of types ‘int’ and ‘float’ to binary ‘operator%’
endMin = e%100.0f;
(Using Netbeans btw)
I tried to just add .0f to each the s/100 and e/100 but the result was the same.
Trying to add the .0f to every other constant obviously doesn't work, why is that? How could I make that work? Is the rounding solution the only other way? (as stated by Irrelevant Elephant)