use OS independent datetime formatting

Author: JuliaSprenger

neo/io/nixio.py

@@ -630,7 +630,7 @@ def write_block(self, block, use_obj_names=False):
         metadata["neo_name"] = neoname
         nixblock.definition = block.description
         if block.rec_datetime:
-            nix_rec_dt = int(block.rec_datetime.strftime("%s"))
+            nix_rec_dt = int(block.rec_datetime.timestamp())
             nixblock.force_created_at(nix_rec_dt)
         if block.file_datetime:
             fdt, annotype = dt_to_nix(block.file_datetime)