When a crypto chart shows 24 hour high and low, does it mean high/low from midnight last night or does it mean the high/low within the last 24 hours from the time you are looking at the chart?
If it refers to midnight, what time zone would it be referencing?
Similarly, when it shows 24 hour volume, is that from midnight last night or is it the volume for the last 24 hours from the time you are viewing the chart?
There is no fixed standard on this. I have seen trading platforms (not limited to crypto) with time zone based time-frame settings; like we must check with them what clock they are using and from that we can assume data being shown are last day's data when it is about last 24 hrs data. Also, some platforms are using running time frame. This is all about exactly last 23 hours 55 minutes data are included and when another five minutes of data occurs, data from 24 hours will be removed for calculations (here refresh time is 5 minutes, if I remember correctly we may set smaller refresh time to get more accurate 24 hrs calculations).
You must check with your exchange or trading terminal or chart provider for the type of calculations they are using. Or you can find it by yourself when extreme market conditions happens like when sudden spikes happens 24 hrs average value will drastically vary from current market price and it will get closer to market price when 24 hrs passed. By close observation, we can find what kind of trading environment we are using.