diff --git a/NAMESPACE b/NAMESPACE
index 1362b15cd..b0165a2f1 100644
--- a/NAMESPACE
+++ b/NAMESPACE
@@ -95,6 +95,7 @@ importFrom(checkmate,check_atomic)
 importFrom(checkmate,check_data_frame)
 importFrom(checkmate,check_names)
 importFrom(checkmate,expect_class)
+importFrom(checkmate,testInt)
 importFrom(checkmate,test_set_equal)
 importFrom(checkmate,test_subset)
 importFrom(checkmate,vname)
diff --git a/R/epiprocess.R b/R/epiprocess.R
index dd7df87a8..bea15b1d6 100644
--- a/R/epiprocess.R
+++ b/R/epiprocess.R
@@ -8,6 +8,7 @@
 #'             assert_logical assert_list assert_character assert_class
 #'             assert_int assert_numeric check_data_frame vname check_atomic
 #'             anyInfinite test_subset test_set_equal checkInt expect_class
+#'             testInt
 #' @importFrom cli cli_abort cli_warn
 #' @importFrom rlang %||%
 #' @name epiprocess
diff --git a/R/grouped_epi_archive.R b/R/grouped_epi_archive.R
index c6326751c..608ce27f5 100644
--- a/R/grouped_epi_archive.R
+++ b/R/grouped_epi_archive.R
@@ -204,10 +204,16 @@ ungroup.grouped_epi_archive <- function(x, ...) {
 #' @importFrom rlang !! !!! enquo quo_is_missing enquos is_quosure sym syms
 #'  env missing_arg
 #' @export
-epix_slide.grouped_epi_archive <- function(x, f, ..., before, ref_time_values,
-                                           time_step, new_col_name = "slide_value",
-                                           as_list_col = FALSE, names_sep = "_",
-                                           all_versions = FALSE) {
+epix_slide.grouped_epi_archive <- function(
+    x,
+    f,
+    ...,
+    before = Inf,
+    ref_time_values = NULL,
+    new_col_name = "slide_value",
+    as_list_col = FALSE,
+    names_sep = "_",
+    all_versions = FALSE) {
   # Perform some deprecated argument checks without using `<param> =
   # deprecated()` in the function signature, because they are from
   # early development versions and much more likely to be clutter than
@@ -231,7 +237,7 @@ epix_slide.grouped_epi_archive <- function(x, f, ..., before, ref_time_values,
         ", class = "epiprocess__epix_slide_all_rows_parameter_deprecated")
   }
 
-  if (missing(ref_time_values)) {
+  if (is.null(ref_time_values)) {
     ref_time_values <- epix_slide_ref_time_values_default(x$private$ungrouped)
   } else {
     assert_numeric(ref_time_values, min.len = 1L, null.ok = FALSE, any.missing = FALSE)
@@ -247,19 +253,9 @@ epix_slide.grouped_epi_archive <- function(x, f, ..., before, ref_time_values,
   }
 
   # Validate and pre-process `before`:
-  if (missing(before)) {
-    cli_abort("`before` is required (and must be passed by name);
-                if you did not want to apply a sliding window but rather
-                to map `epix_as_of` and `f` across various `ref_time_values`,
-                pass a large `before` value (e.g., if time steps are days,
-                `before=365000`).")
+  if (!(anyInfinite(before) || testInt(before, lower = 0L, null.ok = FALSE, na.ok = FALSE))) {
+    cli_abort("`before` must be a non-negative integer or Inf.")
   }
-  before <- vctrs::vec_cast(before, integer())
-  assert_int(before, lower = 0L, null.ok = FALSE, na.ok = FALSE)
-
-  # If a custom time step is specified, then redefine units
-
-  if (!missing(time_step)) before <- time_step(before)
 
   # Symbolize column name
   new_col <- sym(new_col_name)
diff --git a/R/methods-epi_archive.R b/R/methods-epi_archive.R
index 891cc064a..7a0334038 100644
--- a/R/methods-epi_archive.R
+++ b/R/methods-epi_archive.R
@@ -790,9 +790,8 @@ epix_slide <- function(
     x,
     f,
     ...,
-    before,
-    ref_time_values,
-    time_step,
+    before = Inf,
+    ref_time_values = NULL,
     new_col_name = "slide_value",
     as_list_col = FALSE,
     names_sep = "_",
@@ -803,10 +802,16 @@ epix_slide <- function(
 
 #' @rdname epix_slide
 #' @export
-epix_slide.epi_archive <- function(x, f, ..., before, ref_time_values,
-                                   time_step, new_col_name = "slide_value",
-                                   as_list_col = FALSE, names_sep = "_",
-                                   all_versions = FALSE) {
+epix_slide.epi_archive <- function(
+    x,
+    f,
+    ...,
+    before = Inf,
+    ref_time_values = NULL,
+    new_col_name = "slide_value",
+    as_list_col = FALSE,
+    names_sep = "_",
+    all_versions = FALSE) {
   # For an "ungrouped" slide, treat all rows as belonging to one big
   # group (group by 0 vars), like `dplyr::summarize`, and let the
   # resulting `grouped_epi_archive` handle the slide:
@@ -814,8 +819,7 @@ epix_slide.epi_archive <- function(x, f, ..., before, ref_time_values,
     group_by(x),
     f,
     ...,
-    before = before, ref_time_values = ref_time_values,
-    time_step = time_step, new_col_name = new_col_name,
+    before = before, ref_time_values = ref_time_values, new_col_name = new_col_name,
     as_list_col = as_list_col, names_sep = names_sep,
     all_versions = all_versions
   ) %>%
diff --git a/man/epix_slide.Rd b/man/epix_slide.Rd
index c8f09594b..f8e0b914f 100644
--- a/man/epix_slide.Rd
+++ b/man/epix_slide.Rd
@@ -10,9 +10,8 @@ epix_slide(
   x,
   f,
   ...,
-  before,
-  ref_time_values,
-  time_step,
+  before = Inf,
+  ref_time_values = NULL,
   new_col_name = "slide_value",
   as_list_col = FALSE,
   names_sep = "_",
@@ -23,9 +22,8 @@ epix_slide(
   x,
   f,
   ...,
-  before,
-  ref_time_values,
-  time_step,
+  before = Inf,
+  ref_time_values = NULL,
   new_col_name = "slide_value",
   as_list_col = FALSE,
   names_sep = "_",
@@ -36,9 +34,8 @@ epix_slide(
   x,
   f,
   ...,
-  before,
-  ref_time_values,
-  time_step,
+  before = Inf,
+  ref_time_values = NULL,
   new_col_name = "slide_value",
   as_list_col = FALSE,
   names_sep = "_",
@@ -97,12 +94,6 @@ set to a regularly-spaced sequence of values set to cover the range of
 \code{version}s in the \code{DT} plus the \code{versions_end}; the spacing of values will
 be guessed (using the GCD of the skips between values).}
 
-\item{time_step}{Optional function used to define the meaning of one time
-step, which if specified, overrides the default choice based on the
-\code{time_value} column. This function must take a positive integer and return
-an object of class \code{lubridate::period}. For example, we can use \code{time_step = lubridate::hours} in order to set the time step to be one hour (this
-would only be meaningful if \code{time_value} is of class \code{POSIXct}).}
-
 \item{new_col_name}{String indicating the name of the new column that will
 contain the derivative values. Default is "slide_value"; note that setting
 \code{new_col_name} equal to an existing column name will overwrite this column.}
@@ -124,6 +115,12 @@ from \code{new_col_name} entirely.}
 \code{ref_time_value - before} and \code{ref_time_value}. Otherwise, \code{f} will be
 passed only the most recent \code{version} for every unique \code{time_value}.
 Default is \code{FALSE}.}
+
+\item{time_step}{Optional function used to define the meaning of one time
+step, which if specified, overrides the default choice based on the
+\code{time_value} column. This function must take a positive integer and return
+an object of class \code{lubridate::period}. For example, we can use \code{time_step = lubridate::hours} in order to set the time step to be one hour (this
+would only be meaningful if \code{time_value} is of class \code{POSIXct}).}
 }
 \value{
 A tibble whose columns are: the grouping variables, \code{time_value},
diff --git a/tests/testthat/test-epix_slide.R b/tests/testthat/test-epix_slide.R
index a5b72cbfa..8e75fa9bf 100644
--- a/tests/testthat/test-epix_slide.R
+++ b/tests/testthat/test-epix_slide.R
@@ -174,30 +174,18 @@ test_that("epix_slide works as intended with `as_list_col=TRUE`", {
 })
 
 test_that("epix_slide `before` validation works", {
-  expect_error(
-    xx %>% epix_slide(f = ~ sum(.x$binary)),
-    "`before` is required"
-  )
   expect_error(
     xx %>% epix_slide(f = ~ sum(.x$binary), before = NA),
-    "Assertion on 'before' failed: May not be NA"
+    "`before` must be a non-negative integer or Inf."
   )
   expect_error(
     xx %>% epix_slide(f = ~ sum(.x$binary), before = -1),
-    "Assertion on 'before' failed: Element 1 is not >= 0"
+    "`before` must be a non-negative integer or Inf."
   )
   expect_error(
     xx %>% epix_slide(f = ~ sum(.x$binary), before = 1.5),
-    regexp = "before",
-    class = "vctrs_error_incompatible_type"
-  )
-  # We might want to allow this at some point (issue #219):
-  expect_error(
-    xx %>% epix_slide(f = ~ sum(.x$binary), before = Inf),
-    regexp = "before",
-    class = "vctrs_error_incompatible_type"
+    "`before` must be a non-negative integer or Inf."
   )
-  expect_error(xx %>% epix_slide(f = ~ sum(.x$binary)), "`before` is required")
   # These `before` values should be accepted:
   expect_error(
     xx %>% epix_slide(f = ~ sum(.x$binary), before = 0),