Add date calculation support for PHP 8

[TheWitness]

Describe the bug

Stricter typing in PHP 8.1.2 generates warnings when using floats on functions that require an integer. In the past, these conversions happened silently, but in PHP 8.1.2 they generate an error. In PHP 8.1.2, the Cacti get_daysfromtime() generates such warnings.

Expected behavior

Less warnings in Cacti

[TheWitness]

Commit is here: 44bf278