Support watchOS 32bit

[swhitty]

#14 added ability to encode/decode dates to integers using .millisecondsSince1970 and .secondsSince1970 but on arm64_32 environments (some watchOS) this overflows because Int is only 32 bits.

This PR updates the encoding/decoding to explicitly use Int64 for this field.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.74%. Comparing base (d5ca756) to head (f7bf4ed).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #18   +/-   ##
=======================================
  Coverage   99.74%   99.74%           
=======================================
  Files           3        3           
  Lines         775      775           
=======================================
  Hits          773      773           
  Misses          2        2           

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