I found this while investigating non determinstic builds across different machines. It took me a while because I looked at the checksums before realizing the date fields (appearing much later in the font file) were different. Then I realized it was a date but strangely it wasn't changing with faketime and eventually I figured macTime(hi, lo) was returning -1, leaving hi and lo uninitialised.
Hope you'll find this fix appropriate, otherwise cosider this as a simple bug report.