diff --git a/r-package/R/accessibility.R b/r-package/R/accessibility.R
index 7e755c9f..ad85b534 100644
--- a/r-package/R/accessibility.R
+++ b/r-package/R/accessibility.R
@@ -128,7 +128,7 @@ accessibility <- function(r5r_network,
                           opportunities_colnames = "opportunities",
                           mode = "WALK",
                           mode_egress = "WALK",
-                          departure_datetime = Sys.time(),
+                          departure_datetime = NULL,
                           time_window = 10L,
                           percentiles = 50L,
                           decay_function = "step",
@@ -179,7 +179,7 @@ accessibility <- function(r5r_network,
   destinations <- assign_points_input(destinations, "destinations")
   opportunities <- assign_opportunities(destinations, opportunities_colnames)
   mode_list <- assign_mode(mode, mode_egress)
-  departure <- assign_departure(departure_datetime)
+  departure <- assign_departure(departure_datetime, mode_list)
 
   # check availability of transit services on the selected date
   if (mode_list$transit_mode %like% 'TRANSIT|TRAM|SUBWAY|RAIL|BUS|CABLE_CAR|GONDOLA|FUNICULAR') {
diff --git a/r-package/R/arrival_travel_time_matrix.R b/r-package/R/arrival_travel_time_matrix.R
index 012028aa..501a8155 100644
--- a/r-package/R/arrival_travel_time_matrix.R
+++ b/r-package/R/arrival_travel_time_matrix.R
@@ -86,7 +86,7 @@ arrival_travel_time_matrix <- function(r5r_network,
                                        destinations,
                                        mode = "WALK",
                                        mode_egress = "WALK",
-                                       arrival_datetime = Sys.time(),
+                                       arrival_datetime,
                                        breakdown = FALSE,
                                        max_walk_time = Inf,
                                        max_bike_time = Inf,
@@ -133,6 +133,11 @@ arrival_travel_time_matrix <- function(r5r_network,
   mode_list <- assign_mode(mode, mode_egress)
 
   # calculate departure datetime
+  checkmate::assert(
+    !is.null(arrival_datetime),
+    .var.name = "arrival_datetime",
+    msg = "You must pass an arrival_datetime to use arrival_travel_time_matrix"
+  )
   departure_datetime <- arrival_datetime - as.difftime(max_trip_duration, units = "mins")
   departure <- assign_departure(departure_datetime)
 
diff --git a/r-package/R/assign.R b/r-package/R/assign.R
index 150ad33d..4923986c 100644
--- a/r-package/R/assign.R
+++ b/r-package/R/assign.R
@@ -142,7 +142,16 @@ assign_mode <- function(mode, mode_egress, style) {
 #' @family assigning functions
 #'
 #' @keywords internal
-assign_departure <- function(datetime) {
+assign_departure <- function(datetime, mode_list) {
+  if (is.null(datetime)) {
+    if (mode_list$transit_mode == "") {
+      datetime <- Sys.time()
+    } else {
+      stop("When using a transit mode you must pass a departure_datetime", call. = FALSE)
+    }
+  }
+
+
   checkmate::assert_posixct(
     datetime,
     len = 1,
diff --git a/r-package/R/detailed_itineraries.R b/r-package/R/detailed_itineraries.R
index 66c6ba28..30522e8e 100644
--- a/r-package/R/detailed_itineraries.R
+++ b/r-package/R/detailed_itineraries.R
@@ -102,7 +102,7 @@ detailed_itineraries <- function(r5r_network,
                                  destinations,
                                  mode = "WALK",
                                  mode_egress = "WALK",
-                                 departure_datetime = Sys.time(),
+                                 departure_datetime = NULL,
                                  time_window = 10L,
                                  suboptimal_minutes = 0L,
                                  max_walk_time = Inf,
@@ -157,7 +157,7 @@ detailed_itineraries <- function(r5r_network,
   destinations <- od_list$destinations
 
   mode_list <- assign_mode(mode, mode_egress)
-  departure <- assign_departure(departure_datetime)
+  departure <- assign_departure(departure_datetime, mode_list)
 
   # check availability of transit services on the selected date
   if (mode_list$transit_mode %like% 'TRANSIT|TRAM|SUBWAY|RAIL|BUS|CABLE_CAR|GONDOLA|FUNICULAR') {
diff --git a/r-package/R/expanded_travel_time_matrix.R b/r-package/R/expanded_travel_time_matrix.R
index 4131ccbd..f0bd6fa6 100644
--- a/r-package/R/expanded_travel_time_matrix.R
+++ b/r-package/R/expanded_travel_time_matrix.R
@@ -96,7 +96,7 @@ expanded_travel_time_matrix <- function(r5r_network,
                                         destinations,
                                         mode = "WALK",
                                         mode_egress = "WALK",
-                                        departure_datetime = Sys.time(),
+                                        departure_datetime = NULL,
                                         time_window = 10L,
                                         breakdown = FALSE,
                                         max_walk_time = Inf,
@@ -140,7 +140,7 @@ expanded_travel_time_matrix <- function(r5r_network,
   origins <- assign_points_input(origins, "origins")
   destinations <- assign_points_input(destinations, "destinations")
   mode_list <- assign_mode(mode, mode_egress)
-  departure <- assign_departure(departure_datetime)
+  departure <- assign_departure(departure_datetime, mode_list)
 
   # check availability of transit services on the selected date
   if (mode_list$transit_mode %like% 'TRANSIT|TRAM|SUBWAY|RAIL|BUS|CABLE_CAR|GONDOLA|FUNICULAR') {
diff --git a/r-package/R/isochrone.R b/r-package/R/isochrone.R
index 9244f4b7..d7515d31 100644
--- a/r-package/R/isochrone.R
+++ b/r-package/R/isochrone.R
@@ -167,7 +167,7 @@ isochrone <- function(r5r_network,
                       mode_egress = "walk",
                       cutoffs = c(0, 15, 30),
                       zoom = 10,
-                      departure_datetime = Sys.time(),
+                      departure_datetime = NULL,
                       polygon_output = TRUE,
                       time_window = 10L,
                       max_walk_time = Inf,
diff --git a/r-package/R/pareto_frontier.R b/r-package/R/pareto_frontier.R
index 26712211..4c7f753a 100644
--- a/r-package/R/pareto_frontier.R
+++ b/r-package/R/pareto_frontier.R
@@ -93,7 +93,7 @@ pareto_frontier <- function(r5r_network,
                             destinations,
                             mode = c("WALK", "TRANSIT"),
                             mode_egress = "WALK",
-                            departure_datetime = Sys.time(),
+                            departure_datetime = NULL,
                             time_window = 10L,
                             percentiles = 50L,
                             max_walk_time = Inf,
@@ -135,7 +135,7 @@ pareto_frontier <- function(r5r_network,
   origins <- assign_points_input(origins, "origins")
   destinations <- assign_points_input(destinations, "destinations")
   mode_list <- assign_mode(mode, mode_egress)
-  departure <- assign_departure(departure_datetime)
+  departure <- assign_departure(departure_datetime, mode_list)
 
   # check availability of transit services on the selected date
   if (mode_list$transit_mode %like% 'TRANSIT|TRAM|SUBWAY|RAIL|BUS|CABLE_CAR|GONDOLA|FUNICULAR') {
diff --git a/r-package/R/travel_time_matrix.R b/r-package/R/travel_time_matrix.R
index 1d189275..07424e19 100644
--- a/r-package/R/travel_time_matrix.R
+++ b/r-package/R/travel_time_matrix.R
@@ -117,7 +117,7 @@ travel_time_matrix <- function(r5r_network,
                                destinations,
                                mode = "WALK",
                                mode_egress = "WALK",
-                               departure_datetime = Sys.time(),
+                               departure_datetime = NULL,
                                time_window = 10L,
                                percentiles = 50L,
                                max_walk_time = Inf,
@@ -163,7 +163,7 @@ travel_time_matrix <- function(r5r_network,
   origins <- assign_points_input(origins, "origins")
   destinations <- assign_points_input(destinations, "destinations")
   mode_list <- assign_mode(mode, mode_egress)
-  departure <- assign_departure(departure_datetime)
+  departure <- assign_departure(departure_datetime, mode_list)
 
   # check availability of transit services on the selected date
   if (mode_list$transit_mode %like% 'TRANSIT|TRAM|SUBWAY|RAIL|BUS|CABLE_CAR|GONDOLA|FUNICULAR') {
diff --git a/r-package/man/accessibility.Rd b/r-package/man/accessibility.Rd
index c5915c93..17062f93 100644
--- a/r-package/man/accessibility.Rd
+++ b/r-package/man/accessibility.Rd
@@ -12,7 +12,7 @@ accessibility(
   opportunities_colnames = "opportunities",
   mode = "WALK",
   mode_egress = "WALK",
-  departure_datetime = Sys.time(),
+  departure_datetime = NULL,
   time_window = 10L,
   percentiles = 50L,
   decay_function = "step",
diff --git a/r-package/man/arrival_travel_time_matrix.Rd b/r-package/man/arrival_travel_time_matrix.Rd
index c7b5b9fd..6e737118 100644
--- a/r-package/man/arrival_travel_time_matrix.Rd
+++ b/r-package/man/arrival_travel_time_matrix.Rd
@@ -12,7 +12,7 @@ arrival_travel_time_matrix(
   destinations,
   mode = "WALK",
   mode_egress = "WALK",
-  arrival_datetime = Sys.time(),
+  arrival_datetime,
   breakdown = FALSE,
   max_walk_time = Inf,
   max_bike_time = Inf,
diff --git a/r-package/man/assign_departure.Rd b/r-package/man/assign_departure.Rd
index 33af5ec1..a9ab30a2 100644
--- a/r-package/man/assign_departure.Rd
+++ b/r-package/man/assign_departure.Rd
@@ -4,7 +4,7 @@
 \alias{assign_departure}
 \title{Check and convert POSIXct objects to strings}
 \usage{
-assign_departure(datetime)
+assign_departure(datetime, mode_list)
 }
 \arguments{
 \item{datetime}{An object of POSIXct class.}
diff --git a/r-package/man/detailed_itineraries.Rd b/r-package/man/detailed_itineraries.Rd
index e110d9c2..3f5a116c 100644
--- a/r-package/man/detailed_itineraries.Rd
+++ b/r-package/man/detailed_itineraries.Rd
@@ -11,7 +11,7 @@ detailed_itineraries(
   destinations,
   mode = "WALK",
   mode_egress = "WALK",
-  departure_datetime = Sys.time(),
+  departure_datetime = NULL,
   time_window = 10L,
   suboptimal_minutes = 0L,
   max_walk_time = Inf,
diff --git a/r-package/man/expanded_travel_time_matrix.Rd b/r-package/man/expanded_travel_time_matrix.Rd
index 79b58cef..eeeea8dc 100644
--- a/r-package/man/expanded_travel_time_matrix.Rd
+++ b/r-package/man/expanded_travel_time_matrix.Rd
@@ -11,7 +11,7 @@ expanded_travel_time_matrix(
   destinations,
   mode = "WALK",
   mode_egress = "WALK",
-  departure_datetime = Sys.time(),
+  departure_datetime = NULL,
   time_window = 10L,
   breakdown = FALSE,
   max_walk_time = Inf,
diff --git a/r-package/man/isochrone.Rd b/r-package/man/isochrone.Rd
index 544888be..b3f90f40 100644
--- a/r-package/man/isochrone.Rd
+++ b/r-package/man/isochrone.Rd
@@ -12,7 +12,7 @@ isochrone(
   mode_egress = "walk",
   cutoffs = c(0, 15, 30),
   zoom = 10,
-  departure_datetime = Sys.time(),
+  departure_datetime = NULL,
   polygon_output = TRUE,
   time_window = 10L,
   max_walk_time = Inf,
diff --git a/r-package/man/pareto_frontier.Rd b/r-package/man/pareto_frontier.Rd
index fe939272..4ad22094 100644
--- a/r-package/man/pareto_frontier.Rd
+++ b/r-package/man/pareto_frontier.Rd
@@ -11,7 +11,7 @@ pareto_frontier(
   destinations,
   mode = c("WALK", "TRANSIT"),
   mode_egress = "WALK",
-  departure_datetime = Sys.time(),
+  departure_datetime = NULL,
   time_window = 10L,
   percentiles = 50L,
   max_walk_time = Inf,
diff --git a/r-package/man/travel_time_matrix.Rd b/r-package/man/travel_time_matrix.Rd
index 2fd8876e..8cd77800 100644
--- a/r-package/man/travel_time_matrix.Rd
+++ b/r-package/man/travel_time_matrix.Rd
@@ -12,7 +12,7 @@ travel_time_matrix(
   destinations,
   mode = "WALK",
   mode_egress = "WALK",
-  departure_datetime = Sys.time(),
+  departure_datetime = NULL,
   time_window = 10L,
   percentiles = 50L,
   max_walk_time = Inf,