👴🏻hint[old-version]: Freshly built binary reports version is 600,000+ days old🪦 #1468

Description

After building and installing Sapling from source using make install-oss (or build.py), running sl produces a warning claiming the newly installed binary is hundreds of thousands of days old:

hint[old-version]: WARNING! Your version of Sapling is 682491 days old. Please upgrade your installation.
hint[hint-ack]: use 'sl hint --ack old-version' to silence these hints

For example, on a build completed today with version 20261007_015833_2923544b8955, Sapling reports that the version is 682,491 days old (~1,868 years).

Culprit Commit

The issue was introduced in commit 2a6f939 ("build: add build.py") and switched on for make targets in 25b41e7 ("Makefile: use build.py for building").

Cause

In the legacy setup.py, pickversion() formatted versions with a Mercurial compatibility prefix:
4.4.2_YYYYMMDD_HHMMSS_<hash>

Commit 2a6f939 introduced build.py, where auto_version() omitted the 4.4.2_ prefix and outputs:
YYYYMMDD_HHMMSS_<hash>

However, eden/scm/sapling/util.py:versionagedays() still assumes the date is located at index 1 after splitting on _:

parts = remod.split("_", v)
approxbuilddate = datetime.datetime.strptime(parts[1], "%Y%m%d")

Because of the dropped prefix, parts[1] now contains HHMMSS (the build time) rather than YYYYMMDD (the build date). Whenever Python's strptime successfully parses the 6-digit time string with %Y%m%d (treating HHMM as the year and SS as month/day), it resolves to an ancient year (e.g., 0158-03-03), resulting in a calculation of 600,000+ days elapsed.