BUGFIX: make sure calibration file is correct and exists

I've seen some failures in playground PE with v1.3.1 because the calibration path is incorrect for times ending with e.g., 009862 because I truncate the leading zeros.

I switched to using string manipulations to fix this and also added an explicit check that the file exists so that if for some reason the logic gets mangled it will just continue looking back in time.

This is fairly urgent for online PE and so should probably get a bugfix release with just this change as 1.3.2.

cc @soichiro @aaron.zimmerman for online integration/review