This is what I meant.

# create a DateTime object from an ISO timestring
my $isostr = '2021-10-09T11:13:57';    # jd 2459496.96802
my $dt     = DateTime::Format::ISO8601->parse_datetime($isostr);
my @phases = phasehunt($dt->epoch);
$logger->info("@phases");
my $start_epoch = $phases[0];
my $end_epoch   = $phases[4];
my $diff        = $end_epoch - $start_epoch;
my $hard_bottom = int($start_epoch);
$logger->info("integer bottom is $hard_bottom");
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Output:

2021/10/14 13:49:52 INFO 1633518344.15928 1634095655.87543 1634741861.
+07993 1635451604.37235 1636060526.50686 
2021/10/14 13:49:52 INFO integer bottom is 1633518344
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Anything numerical happens over an interval, and paying special attention to the bounds has a profound bearing on whether results are valid. I like to bound an interval with lub and glb integers to check results with familiar tools.

Thanks for your comments, I'm trying to get this written before before the interval has elapsed....