Issue with system.uptime

The system.uptime JavaScript variable is giving unexpected results in current builds.

Given this line:

print("This BBS has been up since " + system.timestr(system.uptime));

In b3cec4b6, result is the current date/time:

This BBS has been up since Fri Apr 03 2026 02:08 pm

In dc7a1772, result is instead:

This BBS has been up since Wed Dec 31 1969 08:02 pm

Chatted with nelgin and Deuce in IRC; they suspect possibly related to #1093 (closed).