Time Elapse Two Datetimes in Hours

Question

I am trying to find time elapsed between two datetimes in hours. Below code attempted by me.

Example: A record is saved and after 2 days we have to calculate the elapsed time in hours i.e. Number of hours after 2 days from the saved datetime. Also hours will be an positive Integer.

if( (DateTime.Now() - SavedTime__c) >= 2 ) // to check if 2 days time has elapsed or not
{
        if(floor(((DateTime.Now() - SavedTime__c)) * 24) > 0) // converting time difference in hours
        {
            CalculatedHours__c = floor((DateTime.Now() - SavedTime__c) * 24);   // Updating the hours value in a field with data type number
        }
        else
        {
        CalculatedHours__c = 0;  // else be zero
      }
    }

Error : Date/time expressions must use Integer or Double or Decimal I have tried conversion but the error persists.

SavedTime__c field : Data Type = Date time

CalculatedHours__c : Data Type = Number

Answer 1

As sfdcfox points out in his comment, you seem to be mixing syntax for formulas and syntax for Apex. I get the impression that you found a formula somewhere and are trying to convert it to Apex (you have variables! USE THEM!). The rest of this answer is framed around the assumption that you're doing this in Apex.

The error itself is because <Datetime> - <Datetime> is not valid in Apex. It is valid in formulas and gives you a result like 2.123 (where 2 is the number of days and .123 is a fractional day down to a precision of seconds), but not in Apex.

So everywhere you have (DateTime.Now() - SavedTime__c) is a syntax error.

Like the error message is telling you, you can only use an Integer, Double, or Decimal when you want to add/subtract from a Datetime. So Datetime.now() - 2, for example, would subtract 2 days.

The Datetime class itself doesn't have any methods that we can use to get a decimal representation of the difference between them. Seems like an odd omission by Salesforce, given that we have daysBetween() on Date, but it doesn't look like Java has a single method to accomplish the same thing either (and I'm less and less surprised by Salesforce's inconsistencies as time goes on).

Anyway, to do this in Apex, you'll need to work with timestamps (number of seconds/milliseconds between the unix epoch of 1970-01-01T00:00:00Z and a given Datetime). The getTime() method gets you that information.

// Difference in # of milliseconds
Long difference = Datetime.now().getTime() - SavedTime__c.getTime();

// There are 1000 milliseconds in a second
// 60 seconds in a minute
// and 60 minutes in an hour
// 1000 * 60 * 60 = 3,600,000

// Division between two Longs will remove the fractional part (i.e. it'll 
//   automatically floor the result)
Long hourDifference = difference / 3600000;

// If you want fractional hours, make either the numerator or denominator a Decimal
//Decimal hourDifference = difference / 3600000.0;

The logic you have to decide between putting the hour difference or 0 into the final field is nonsensical. If the number of days between two datetimes is at least 2, then the number of hours will always be positive. The else block where you have CalculatedHours__c = 0; is unreachable.

Thus, with your current logic, you should simply assign the difference in hours to CalculatedHours__c if that difference is at least 2 days (i.e. 48 hours). You haven't provided enough detail for us to know what should happen when the difference is less than 48 hours.