From Harry Potter and the Cursed Child - Part I:
Scorpius Malfoy is talking with his best friend Albus Potter:
SCORPIUS: So your dad thinks the rumors are true -- I am the son of Voldemort?
ALBUS (nods): His department are currently investigating it.
SCORPIUS: Good. Let them. Sometimes -- sometimes I find myself thinking -- maybe they're true too.
ALBUS: No. They're not true. [...]
Harry Potter and the Cursed Child, Part I - Act Two, Scene Sixteen, Hogwarts Library - Page 143 - Scholastic
Harry Potter and the Cursed Child picks up right at the point where Harry Potter and the Deathly Hallows leaves off, at the epilogue. It is nineteen years post-Hogwarts, and Scorpius Malfoy and Albus Potter are eleven-year-old first-years.
I'm about as good at math as J.K. Rowling (meaning I'm terrible at it!), so correct me if I'm wrong. Voldemort was killed at the Battle of Hogwarts in May of 1998. At least eight years post-Battle had to have passed before Albus and Scorpius were born (I'm estimating the boys were born in 2006). Voldemort would have had to have been alive in 2005/2006 in order to have ...
... fathered Scorpius Malfoy.
As the wizarding world was fully aware of Voldemort's death in 1998, why would anyone even remotely believe that Voldemort is ...
... Scorpius Malfoy's father?
I can understand the possibility of Harry believing the rumor because ...
... his scar hurt, and his scar is Harry's connection to Voldemort.
But other than Harry, it doesn't make sense to me why anyone would believe such a ridiculous rumor. So, why?