You can create a new formula field that outputs the post date if there is one and a date far into the future if there is no post date. Then sort on the formula field.
IF(
{Posting Date & Time},
{Posting Date & Time},
DATETIME_PARSE("2050-01-01", "YYYY-MM-DD")
)
Filtering out the records with no post date so that you don’t see them at all might be another option.