When you play a beatmap with double time or any mod that modifies playback speed, the time in the bottom corners is the normal time of the song duration but it goes by faster. I think it should represent the real time it takes to play the beatmap with the current speed value.
osu!lazer 2020.429.0
For wind-up mod, little bit of algebra needs to be used when calculating the final duration. The resulting algorithm applies to all speed-up mods so this could probably just be slotted in as the calculation for all durations, including no-mod. The algorithm being: final_duration = 2 * original_duration / (initial_speed + final_speed)
Countdown during break time is also affected by speed modifications and also doesn't represent real time.
As a data point, music players (spotify/youtube/whichever) don't change the runtime if you set a custom playback speed, so I'm against this proposal.
I agree with @bdach, Double-Time, Half-Time and other similar mods are more of changing the speed at which the clock advances rather than fully manipulating the displayed beatmap length while in-game, I would say if anything, just display the real-time duration next to the normal duration.
It didn't come to mind that music players too don't show the actual time the media will take to playback at set speed. I changed my mind on this so I'm okay with closing this issue unless someone wants to try adding an algorithm, even as a setting in the option (but that doesn't seem like a good idea either)
I want to add that i made this proposal because if you put a -x:xx time one would think that that is the time left until the end and when the speed is normal it does exactly that. I don't know if there are reasons why other media players don't do it but this was my thought process for this.
This is intentional.
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As a data point, music players (spotify/youtube/whichever) don't change the runtime if you set a custom playback speed, so I'm against this proposal.