I need to record a 27 second song...

If my maths is any good 4 beats to the bar at 120 beats per minute should give (27/60) x (120/4) = 13.5 bars at 120bpm.

Since you can't seem to start a project in the track editor record a little something at random, then switch to the track editor. Use the spanner tool to set the tempo to 120bpm. Add a simple drum loop. Click the jigsaw piece to edit the section length to 14 bars, then split the loop half way through the 14th bar and delete the last section.

I should add I'm no musician, but doing the above gives a piece of music 27 seconds long if you ignore the half bar of silence at the end. Redo the maths if you need a different tempo.

tt2

There is no time measurement tool in GarageBand for iPad. The only way to physically measure the number of seconds in your recording is to export the file and use a different tool. This can be as simple as emailing yourself the exported file, and then opening the exported attachment on your iPad to see how many seconds it reports the file will play for. This is still imprecise however, as the OS will either trim or round off fractions of a second.

Personally I use a free dropbox account, and email my compositions to that. Having installed the necessary software also free from dropbox on my laptop, which has become my desktop, those files get automagically added to the files on that system. You've now wirelessly added your export to a system capable of running something like Audacity (open source/donation appreciated) and can get a measurement down to 1/100 of a second if needed.

Your only other choice are the quite correct mathematics @tt2 provided in his response. My only addition to them is that if you require exactly and only 27 seconds of sound, because you're limited to 4/4 time in GBfI, you must set your tempo to be 240 (the max it can go) and record 27 measures of sound. I tried a number of other combinations which get you so near to 27 secs as to be indistinguishable without the aid of a mechanical measuring device, but you made me curious about how exporting 'dead' space is handled, and that was the only combo that yields the exact specified number of seconds. I've only been using anything remotely like this kind of a program for a month now however, about the same amount of time I've owned an iPad, so there may be some trick of timing I couldn't find with just a couple hours dinking around.

I hope that answers your question because I dug way deeper into that than I ever intended and would hate to think I missed it after all that effort! ;-)

-neondrmr

If we want a precise solution...

Tempo range 40-240 bpm. Part lengths have integer number of bars at 4 beats per bar.

Three solutions:

9 bars @ 80 bpm = 27 seconds

18 bars @ 160 bpm = 27 seconds

27 bars @ 240 bpm = 27 seconds

tt2

Ah nuts! Well it's a good thing there are smart people answerig these question too. :-)