Howdy!
From what I know, there are three ways to check your playtime on the server.
The first is doing /profile and hovering over your Minecraft head that is displayed in that menu. It'll show you the playtime you have in days, aswell as your Mana level, Guide, tag and rank.
The second is doing /playtime, but this command is only available if you have 100+ hours of playtime.
Finally the third, is doing /prestige and going to prestige rank that requires playtime. This one will show you accurate hours (no minutes) of how long you have played the realm for (skyblock). Doing this command can also give you tasks you need to complete to earn cubits.
Hopefully this helps, if you have any questions, feel free to ask!