Here's my process:
The first thing that you need is a clock which is ticking in seconds. I have an iPhone and there's a clock with a sweeping second hand on it. For you kids who can't read a clock face that's not digital, your computer has time and you can set it to show seconds.* I actually use a website https:/time.is on my phone as the numbers are big and it's digital.
Ok, once you have the clock running where you can see it, go to your cpap clock setting. (ResMed engineers are obviously complete MORONS when it comes to clocks. The machine has a cell phone modem in it, and it "phones home" each day. While it's busy tattling on you for the terrible crime of taking your mask off briefly while ignoring all of its own failures to control your SDB, it never bothers to ask the computer on the other side "hey, dude, what time is it?" AND they put the clock setting on the clinical menu like it's some super-duper expert concern) Once you have the setting open, look at your clock and compare it to what date and time your machine thinks it is. If the current time is right before the hour is going to change -- i.e. it's something:58 or something:59 -- wait for the hour to roll over before adjusting. If you are doing this around midnight and the cpap clock shows not only the wrong time but the wrong date -- wait until it's after midnight AND the clock shows the right DATE before changing anything!!! -- on a ResMed machine, at least, it's really easy to end up setting the clock to right before midnight the next day and then you are screwed and it will take two days to get your date fixed because of how the ResMed machine treats noon vs midnight -- it will not let you "set the date to the past."
Been There Done That Got The Tshirt.
Once you are at the clock setting line on the screen at a good time safely away from midnight or the top of any hour, start your time adjusting. Select the minute, roll the knob to the value one minute past the current minute, watch your clock count through the seconds. When you get past something:50, get ready. As the second ticks through :59 to :00, click the knob quickly to set the time exactly to the new minute. If it's right, you can watch the clock and as it changes to the next minute then your cpap clock should roll over at the same time.
Once your cpap clock is set to the right time to the second, you can repeat the process with any other devices (pulse-ox in my case) which don't have accurate clocks in them.
Then when using OSCAR you can just tell it not to worry about clock drift as you've got it under control
If anyone else has a simpler way to keep clocks right, do tell!
*(old tech support tip: always set your computer to show seconds on the clock display. That way, when your computer seems non-responsive, you can glance down at the clock and see what the clock is doing. If you see the seconds frozen or stuttering as opposed to counting up evenly, then that means that the computer as a whole is struggling to keep up, as opposed to the program you are running or the website you are looking at or the network. Oftentimes this is some background process that's grabbing lots of system resources but will be finished in a minute or two, and your first reaction should be to see if you can wait it out and it goes away quickly. Because if you keep hammering on the button you are trying to push, or whatever else you are doing in your computer interaction, then it's just going to make whatever the computer is doing take longer. But, anyway, your computer clock should always show seconds.)