Generate 3 digit alphanumeric DATE INDEX

Question

I have a requirement for a container label, that will have a QR code, part of the code will be a "3-digit date index starting from 1/1/2017. For example, 1/1/2017 is 000 and 1/31/2017 is 00U. This is the date when Content Label is printed or the parts manufacturing date.:" I dont know if this date index code is some standard , but if it is I cannot find it anywhere, I need a way to generate it from within bartender barcode software which has vbscript support for when this custom requirements arise, how can we generate this? I was thinking of creating a string containing 0-10 and a-z and then searching for the nth ocurrence (30 in the example given) would give me the U, but I don't know how to handle the middle and leftmost digits once I reach more than 36, like creating my own 39tecimal (is that even a thing) numerical system.