What is a Dynamic Variant? Dynamic Variants consists of selection variable for a selection screen element for which a formula can be defined. If you want the selection screen to show a date range from today plus 7 days until today plus 70 days, the end used could arrange for this to happen. It will show as a non-changeable value range and it is calculated using the parameters you (or your end user) have passed.
How ? Follow these steps: SDN document.
The use of vatiants has a side effect, which is easily overcome, if you know how. Many reports determine clever default values for its select options and parameters. When a variant is used, the settings on the variant will overwrite the cleverly defined defaults, which could be a reason to stop using the variant. To solve this, simply edit the variant: it can be made to ignore fields, which would leave the report defaults in tact.