filter_date_sources: Select the First or Last Date from Several Sources
In epijim/admiral: ADaM in R Asset Library

Select for each subject the first or last observation with respect to a date from a list of sources.

filter_date_sources(
  sources,
  source_datasets,
  by_vars,
  create_datetime = FALSE,
  subject_keys,
  mode
)

`sources`	Sources A list of `tte_source()` objects is expected.
`source_datasets`	Source datasets A named list of datasets is expected. The `dataset_name` field of `tte_source()` refers to the dataset provided in the list.
`by_vars`	By variables If the parameter is specified, for each by group the observations are selected separately.
`create_datetime`	Create datetime variable? If set to `TRUE`, variables `ADTM` is created. Otherwise, variables `ADT` is created.
`subject_keys`	Variables to uniquely identify a subject A list of symbols created using `vars()` is expected.
`mode`	Selection mode (first or last) If `"first"` is specified, for each subject the first observation with respect to the date is included in the output dataset. If `"last"` is specified, the last observation is included in the output dataset. Permitted Values: `"first"`, `"last"`

The following steps are performed to create the output dataset:

For each source dataset the observations as specified by the filter element are selected. Then for each patient the first or last observation (with respect to date) is selected.
The ADT variable is set to the variable specified by the date element. If the date variable is a datetime variable, only the datepart is copied. If the source variable is a character variable, it is converted to a date. If the date is incomplete, it is imputed as the first possible date.
The CNSR is added and set to the value of the censor element.
The selected observations of all source datasets are combined into a single dataset.
For each patient the first or last observation (with respect to the ADT variable) from the single dataset is selected.