DateTime::Format::Epoch microsecond output uses scientific notation

[fglock]

Summary

DateTime::Format::Epoch 0.16 fails one upstream test under PerlOnJava because a large integer-valued numeric result is stringified in scientific notation instead of decimal notation.

Reproduction

Run the normal DateTime::Format::Epoch 0.16 test suite.

The focused failing test is t/004_nano.t, which constructs a formatter with unit => "microseconds" and formats a DateTime corresponding to 1051470597800000 microseconds since the epoch.

A minimal arithmetic reproducer is:

my $x = 1051470597;
$x *= 1e6;
$x += int(0.8 * 1e6);
print "[$x]\\n";

Expected result

System Perl prints:

[1051470597800000]

The complete upstream suite passes under system Perl: 20 files and 133 tests.

Actual result

PerlOnJava prints:

[1.0514705978e+15]

The CPAN test failure is:

# Failed test '"now" (micro)'
# at t/004_nano.t line 32.
# got: '1.0514705978e+15'
# expected: '1051470597800000'

The archived compatibility run recorded 1 failed test program and 1 failed subtest out of 20 programs and 133 subtests. The focused arithmetic reproducer produces the same scientific notation on both the JVM and interpreter backends.

Likely root cause

DateTime::Format::Epoch maps microseconds to the numeric literal 1e6. With the default integer mode for this unit, Perl preserves the integer-valued result in decimal string form. PerlOnJava promotes or retains the calculation as a floating-point scalar and formats the large integer-valued result using scientific notation.

This is a shared numeric representation/stringification compatibility issue, not a CPAN distribution defect, missing prerequisite, timeout, or native-code dependency.

Acceptance criteria

• Integer-valued arithmetic such as the reproducer stringifies as decimal rather than scientific notation when Perl does so.
• DateTime::Format::Epoch 0.16 passes all 133 upstream tests on both backends.
• Add a project-owned regression test for large integer-valued arithmetic involving a 1e6 multiplier and decimal stringification.
• Preserve scientific notation for values and contexts where standard Perl actually uses it.

Evidence

• CPAN run: 20260918-141920-96054
• Distribution: DateTime-Format-Epoch-0.16
• Failing test: t/004_nano.t, test 8 ("now" (micro))
• System Perl: PASS, 20 files / 133 tests
• PerlOnJava: FAIL, 1 file / 1 subtest
• JVM and interpreter: focused arithmetic reproducer confirmed

Related issue check

Issue #1283 concerns omitted optional arguments becoming explicit undef in DateTime::Format::Builder; it is related only by the DateTime ecosystem and does not cover this numeric-formatting failure. No duplicate issue was found for this distribution, test, or scientific-notation symptom.