This is the case for a shutter speed measurer / timer gadget.  I used an Adafruit Trinket M0, SSD1306 oled I2C screen, and generic 5mm IR emitter and sensor.

Github repo for schematics and code at https://github.com/seanmurf/shutterTimeSensor
Of course could be improved with a PCB to eliminate the handmadeness, and expanded with multiple sensors to measure speed across a film plane, but, this seems to work!
My excellent condition Trip35 1/40s setting measures between 1/37 and 1/40 of a second, so I think we're in the ballpark.

Shutter speed accuracy and high speeds/short duration:
I did make some small effort to 'blinder' the photodiode path to measure a 'small' area of the film surface.  And I'm using compiled c++ (arduino) code and "micros() command.
The code alone can read out result values well faster than 1/10,000s but at high speeds I don't know how much % error may be due to photodiode reaction time, ADC time, code loop time, and the diode mounting and 'blinder'ing.  

I've seen recently shutter timers using a 555 timer, not sure if that would bring better accuracy for high speed shutters.  If going high-speed, would want to investigate more seriously into the arduino "micros()" command accuracy, code loop vs interrupt method, photodiode reaction time, and better lensing/focusing for the IR...
At least I think this is better than some python timing code and a bare photodiode!

"lensCover.stl" intended to be used with https://www.printables.com/model/625992-5mm-led-holde but is not strictly required.  The timer will not be triggered by ambient light, so no need to block ambient light out.