use Time::GPS qw(gps_instant_to_mjd gps_mjd_to_instant);
use Math::BigRat;

my $x = Math::BigRat->new('915717017');
my $mjd = gps_instant_to_mjd($x);
print "$mjd\n"; # 2021871299/43200

use DateTime::Format::Epoch::MJD;
my $dt = DateTime::Format::Epoch::MJD->parse_datetime( $mjd );
print $dt; # 1987-01-06T00:00:00