I have to admit to being curious how you have used that code in the past, AND achieved a correct result. To be frank, what you have posted will not work as you intend, hopefully what you have actually used does work.
Fundamentals : A-B counters do not reset themselves. so your seconds and minutes counters will continue counting past their targets, to +32767, then roll round to -32767, setting the overflow bit, and continuing counting round this circle ad infinitum, but the .DN bit never turns off again. Basically the instruction says "Because I have the .OV bit set, I must, by inference, be .DN
So after the first 60 seconds the seconds counter .DN bit comes on, and counts up the minutes counter. After that, nothing, the minutes counter will never get incremented again, and the seconds counter will hold ridiculous numbers most of the time.
What is actually needed is that each counter's .DN bit, as well as incrementing the next counter, is also used to reset the current counter.
If you are going to post code that you claim to have used successfully, then post a copy of the actual code, not what you thought it was. Your sample code could mislead people. Protrac231 asked for "a good example"....
Finally, is there any need to count downtime if you are taking figures over a set period, in your case 24 hours. What about if the machine is switched off, or a power failure occurs. Unless your PLC has a UPS, it will not accumulate these periods as downtime. Downtime can be easily calculated as Period-UpTime.