Use Fixed Point Arithmetic in Interval Parsing

[tustvold]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

The interval parsing logic added in #3762 makes use of floating point arithmetic to parse intervals. This can potentially lead to truncation and precision loss.

Describe the solution you'd like

The integral and fraction part of the interval_period should be parsed and handled separately, making use of checked multiplication and addition to avoid silent overflow

Describe alternatives you've considered

Additional context

[mr-brobot]

FYI I'm working on this now and expect to have a PR open over the weekend. :)