I definitely don't spend a lot of time at births timing contractions (I don't spend a lot of time at births looking at the clock, period), since most of what you need to know about labor progress you can tell more from looking and listening than you can by timing. But there is the occasional situation where it's helpful to do a little contraction tracking, especially when getting ready to pass along information to the midwife or doctor. I found it helpful in dark, quiet room while the mom was resting with her eyes closed. There were no lights and no light-up clocks (other than my phone), but I felt like her contractions were lasting surprisingly long. I turned my screen brightness all the way down and was able to just hit the button silently whenever I heard her start or finish a contraction, and pass along the information to her midwife. This app is simple, free, and does exactly what you want it to: show duration and frequency of contractions.
I've written about this app before, and now it's on the Android Market proper! Quick and easy look-up for medication safety & breastfeeding, from the comprehensive database maintained by the NIH. I make a point of using it and demoing it in front of the medical providers who should know about it - already got an ER doc to download it this way!