Decimal hour shows 4 min 60 seconds not 5 minutes - swift

Question

I think the title says it all but let me go in to it a bit more.
I am storing decimal hours in a DB every 5 minutes while a user uses my app. I am having a bit of trouble displaying the correct time for some of the 5 min blocks in that it shows 4 min and 60 seconds. Every 3rd 5 min block shows this.
So something like this
4:60
10:00
15:00
19:60
Some even have n:59 instead of n:00
Part of the code is:
let hours = Int(floor(decimalHour))
let mins = Int(floor(decimalHour * 60) % 60)
let secs = Int(floor(decimalHour * 3600) % 60)
Any suggestions what I could be doing wrong?

Answer 1

Binary floating point number can not represent all numbers
exactly, therefore your code is prone to rounding errors.
Example (Swift 2):
let decimalHour = 1.0 + 5.0/60.0
print(decimalHour.debugDescription) // 1.0833333333333333
print(floor(decimalHour * 3600)) // 3899.0
let hours = Int(floor(decimalHour))
let mins = Int(floor(decimalHour * 60) % 60)
let secs = Int(floor(decimalHour * 3600) % 60)
print(hours, mins, secs) // 1 5 59
The actual number stored in decimalHour is slightly less than 1 + 5/60, and therefore the seconds are calculated wrongly.
(Also note that you cannot use % with floating point numbers
in Swift 3, compare What does "% is unavailable: Use truncatingRemainder instead" mean?.)
As already said in the comments, a better approach would be
to store the duration as an integer (number of seconds).
If that is not possible, round the floating point number to
a the number of seconds and then continue with pure integer
arithmetic. Example (works with Swift 2+3):
let decimalHour = 1.0 + 5.0/60.0
let totalSeconds = lrint(decimalHour * 3600) // round to seconds
let hours = totalSeconds / 3600
let mins = (totalSeconds % 3600) / 60
let secs = totalSeconds % 60
print(hours, mins, secs) // 1 5 0