mtime fix

commit 87dab97e098733dae83a940b2a2b3e6060c21ae2
Author: Rahul Sinha <rahul.sinha@mpimet.mpg.de>
Date:   Tue Jul 22 20:10:20 2014 +0200

Fix: mtime - Bug #5037. Type int too small in recently introduced
numberOf400YearPeriods. Changing to int64_t