Rotating timetable issues

Welcome to the forum.

Is this a 365 version of Excel? If so, please add this to your forum profile.

Sorry - I can't find the 'very long' if statetemnt that you have tried. And I don't understand what this means:

It's fine if the lessons are in order 1-20 but when these rotate eg 4-20,1-3

Not at all clear what's required here. I think you are forgetting that we know nothing about the fundamentals of your timetable, how it works or what its contstraints are (and I am a former timetabler).

although answers that work with older versions welcome too TIA

It'll be one or the other - so will 365 work across the board for you or not?

Sorry it’s not clear - on the sample sheet I have a dynamic list of groups1-10 (although this can go up to 20) each group can have a lesson from 15mins up to 1 hour. I have sorted a rotating slot for each group over 12 weeks.

I am now trying to work out how to write a formula that will calculate the start time of each groups lesson.

If the group lessons are in order eg gp1 then gp 2 etc then that’s ok but when the lessons are all different lengths of the order changes eg gp4 is first then gp5 etc I am stuck.

The long nested ifs I started but even for only 3 groups this is long and for up to 20 groups this will be unwieldy (I have only started this in cells s10-s12)

Hope this explains is a bit better
If the numbers are in numerical order it’s fine but I can work out

Well, I think this may do what you want, if I have understood correctly:

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It has to be copied across, but not down. I may be able to make it spill both ways, if it's what you need. If so, I'll have another look tomorrow.

OK. That makes sense, I think. I’m offline now (Strictly!), but will look again in the morning.

This fixes the time shift problem:

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I'll see if I can get it to spill across as well as down, although this isn't strictly necessary.

And here's the fully spilling version:

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Glad to have helped.

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Icing on the cake - a solution that requires no helper columns:

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You're welcome.

That's up to you - well done!