diff --git a/DESCRIPTION b/DESCRIPTION
index 544bd38..9aee150 100644
--- a/DESCRIPTION
+++ b/DESCRIPTION
@@ -16,6 +16,7 @@ Imports:
     fs,
     jsonlite,
     logger,
+    lubridate,
     mgcv,
     mime,
     plumber,
diff --git a/R/api.R b/R/api.R
index 17549fa..c18485c 100644
--- a/R/api.R
+++ b/R/api.R
@@ -46,10 +46,28 @@ target_post_dataset <- function(req, res) {
     return(bad_request_response(msg))
   }
 
+  if (suppressWarnings(all(is.na(as.numeric(file_body[, xcol]))))) {
+    xtype <- "date"
+    suppressWarnings({
+      file_body[, xcol] <- parse_date(file_body[, xcol])
+    })
+    if (all(is.na(file_body[, xcol]))) {
+      res$status <- 400L
+      msg <- paste("Invalid x column values:",
+                   "these should be numbers or dates in a standard format.")
+      return(bad_request_response(msg))
+    }
+    logger::log_info("Detected date values in x column")
+  } else {
+    logger::log_info("Detected numeric values in x column")
+    xtype <- "number"
+  }
+
   logger::log_info(paste("Saving dataset", filename, "to disk"))
   dir.create(path, recursive = TRUE)
   utils::write.csv(file_body, file.path(path, "data"), row.names = FALSE)
   write(xcol, file.path(path, "xcol"))
+  write(xtype, file.path(path, "xtype"))
   response_success(jsonlite::unbox(filename))
 }
 
@@ -72,7 +90,7 @@ target_get_dataset <- function(name, req) {
   variables <- list()
   for (col in cols) {
     lvls <- unique(dat[, col])
-    if (length(lvls) < 12) {
+    if (length(lvls) < 25) {
       variables[[col]] <- list(name = jsonlite::unbox(col), levels = lvls)
     }
   }
@@ -100,6 +118,7 @@ target_get_trace <- function(name,
   dataset <- read_dataset(req, name, scale)
   dat <- dataset$data
   xcol <- dataset$xcol
+  xtype <- dataset$xtype
   cols <- colnames(dat)
   if (!is.null(filter)) {
     filters <- strsplit(filter, "+", fixed = TRUE)[[1]]
@@ -117,6 +136,7 @@ target_get_trace <- function(name,
     return(lapply(seq_along(groups), function(i) {
       model <- with_warnings(model_out(groups[[i]],
                                        xcol = xcol,
+                                       xtype = xtype,
                                        method = method,
                                        span = span,
                                        k = k))
@@ -129,6 +149,7 @@ target_get_trace <- function(name,
     logger::log_info("Returning single trace")
     model <- with_warnings(model_out(dat,
                                      xcol = xcol,
+                                     xtype = xtype,
                                      method = method,
                                      span = span,
                                      k = k))
@@ -157,10 +178,19 @@ read_dataset <- function(req, name, scale) {
     dat$value <- log2(dat$value)
   }
   xcol <- readLines(file.path(path, "xcol"))
-  list(data = dat, xcol = xcol)
+  xtype <- readLines(file.path(path, "xtype"))
+  logger::log_info("Parsing x column values")
+  if (xtype == "date") {
+    dat[, xcol] <- as.Date(dat[, xcol], origin = "1970-01-01")
+  }
+  list(data = dat, xcol = xcol, xtype = xtype)
 }
 
-model_out <- function(dat, xcol, method = "auto", span = 0.75, k = 10) {
+model_out <- function(dat, xcol,
+                           xtype = "number",
+                           method = "auto",
+                           span = 0.75,
+                           k = 10) {
   n <- nrow(dat)
   if (n == 0) {
     return(list(x = list(), y = list()))
@@ -170,13 +200,16 @@ model_out <- function(dat, xcol, method = "auto", span = 0.75, k = 10) {
     m <- mgcv::gam(eval(parse(text = fmla)),
                    data = dat, method = "REML")
   } else {
-    fmla <- sprintf("value ~ %s", xcol)
+    fmla <- sprintf("value ~ as.numeric(%s)", xcol)
     m <- stats::loess(fmla, data = dat, span = span)
   }
   range <- range(dat[, xcol], na.rm = TRUE)
   xseq <- range[1]:range[2]
   xdf <- tibble::tibble(xcol = xseq)
   names(xdf) <- xcol
+  if (xtype == "date") {
+    xseq <- format(as.Date(xseq, origin = "1970-01-01"))
+  }
   list(x = xseq, y = stats::predict(m, xdf))
 }
 
diff --git a/R/utils.R b/R/utils.R
index f0b6a3c..d09e7c9 100644
--- a/R/utils.R
+++ b/R/utils.R
@@ -22,3 +22,8 @@ validate_scale <- function(scale) {
     )
   }
 }
+
+parse_date <- function(dat) {
+  as.Date(lubridate::parse_date_time(dat,
+                                     c("dmy", "mdy", "ymd", "ydm")))
+}
diff --git a/docker/Dockerfile b/docker/Dockerfile
index a622ec4..883dd43 100644
--- a/docker/Dockerfile
+++ b/docker/Dockerfile
@@ -17,6 +17,7 @@ RUN install_packages --repo=https://mrc-ide.r-universe.dev \
         docopt \
         fs \
         logger \
+        lubridate \
         jsonlite \
         jsonvalidate \
         plumber \
diff --git a/inst/schema/DataSeries.schema.json b/inst/schema/DataSeries.schema.json
index 15d17f9..bc642f4 100644
--- a/inst/schema/DataSeries.schema.json
+++ b/inst/schema/DataSeries.schema.json
@@ -13,7 +13,7 @@
           "x": {
             "type": "array",
             "items": {
-              "type": "number"
+              "type": ["number", "string"]
             }
           },
           "y": {
@@ -32,7 +32,7 @@
           "x": {
             "type": "array",
             "items": {
-              "type": "number"
+              "type": ["number", "string"]
             }
           },
           "y": {
diff --git a/inst/schema/Variable.schema.json b/inst/schema/Variable.schema.json
index b963a79..2edc88f 100644
--- a/inst/schema/Variable.schema.json
+++ b/inst/schema/Variable.schema.json
@@ -8,7 +8,7 @@
     "levels": {
       "type": "array",
       "items": {
-        "type": "string"
+        "type": ["string",  "number", "null"]
       }
     }
   },
diff --git a/tests/testthat/helper.R b/tests/testthat/helper.R
index de16728..0ea6413 100644
--- a/tests/testthat/helper.R
+++ b/tests/testthat/helper.R
@@ -22,6 +22,7 @@ local_add_dataset <- function(dat, name, session = session_id, env = parent.fram
   dir.create(filepath, recursive = TRUE)
   write.csv(dat, file.path(filepath, "data"), row.names = FALSE)
   write("day", file.path(filepath, "xcol"))
+  write("number", file.path(filepath, "xtype"))
   withr::defer(fs::dir_delete(filepath), envir = env)
   name
 }
diff --git a/tests/testthat/test-dates.R b/tests/testthat/test-dates.R
new file mode 100644
index 0000000..11f7d05
--- /dev/null
+++ b/tests/testthat/test-dates.R
@@ -0,0 +1,83 @@
+test_that("d/m/y", {
+  router <- build_routes(cookie_key)
+  request <- local_POST_dataset_request(data.frame(biomarker = "ab",
+                                                   day = c("01/01/2024",
+                                                           "02/01/2024"),
+                                                   value = 1:10),
+                                        "testdataset",
+                                        cookie = cookie)
+  upload_res <- router$call(request)
+  expect_equal(upload_res$status, 200)
+
+  res <- router$call(make_req("GET",
+                              "/dataset/testdataset/trace/ab/",
+                              HTTP_COOKIE = cookie))
+
+  expect_equal(res$status, 200)
+  body <- jsonlite::fromJSON(res$body)
+  data <- body$data
+  expect_equal(unlist(data$raw[1, "x"]), rep(c("2024-01-01", "2024-01-02"), 5))
+})
+
+test_that("d-m-y", {
+  router <- build_routes(cookie_key)
+  request <- local_POST_dataset_request(data.frame(biomarker = "ab",
+                                                   day = c("01-01-2024",
+                                                           "02-01-2024"),
+                                                   value = 1:10),
+                                        "testdataset",
+                                        cookie = cookie)
+  upload_res <- router$call(request)
+  expect_equal(upload_res$status, 200)
+
+  res <- router$call(make_req("GET",
+                              "/dataset/testdataset/trace/ab/",
+                              HTTP_COOKIE = cookie))
+
+  expect_equal(res$status, 200)
+  body <- jsonlite::fromJSON(res$body)
+  data <- body$data
+  expect_equal(unlist(data$raw[1, "x"]), rep(c("2024-01-01", "2024-01-02"), 5))
+})
+
+test_that("y-m-d", {
+  router <- build_routes(cookie_key)
+  request <- local_POST_dataset_request(data.frame(biomarker = "ab",
+                                                   day = c("2024-01-13",
+                                                           "2024-01-14"),
+                                                   value = 1:10),
+                                        "testdataset",
+                                        cookie = cookie)
+  upload_res <- router$call(request)
+  expect_equal(upload_res$status, 200)
+
+  res <- router$call(make_req("GET",
+                              "/dataset/testdataset/trace/ab/",
+                              HTTP_COOKIE = cookie))
+
+  expect_equal(res$status, 200)
+  body <- jsonlite::fromJSON(res$body)
+  data <- body$data
+  expect_equal(unlist(data$raw[1, "x"]), rep(c("2024-01-13", "2024-01-14"), 5))
+})
+
+test_that("y/d/m", {
+  router <- build_routes(cookie_key)
+  request <- local_POST_dataset_request(data.frame(biomarker = "ab",
+                                                   day = c("2024/14/01",
+                                                           "2024/15/01"),
+                                                   value = 1:10),
+                                        "testdataset",
+                                        cookie = cookie)
+  upload_res <- router$call(request)
+  expect_equal(upload_res$status, 200)
+
+  res <- router$call(make_req("GET",
+                              "/dataset/testdataset/trace/ab/",
+                              HTTP_COOKIE = cookie))
+
+  expect_equal(res$status, 200)
+  body <- jsonlite::fromJSON(res$body)
+  data <- body$data
+  expect_equal(unlist(data$raw[1, "x"]), rep(c("2024-01-14", "2024-01-15"), 5))
+})
diff --git a/tests/testthat/test-model.R b/tests/testthat/test-model.R
index 61ed0b6..7310684 100644
--- a/tests/testthat/test-model.R
+++ b/tests/testthat/test-model.R
@@ -66,3 +66,19 @@ test_that("model uses loess options", {
 
   expect_true(all(res$y == expected))
 })
+
+test_that("model can handle dates", {
+  dates <- sapply(1:50, function(x) as.Date(2 * x, origin = "2023-01-01"))
+  full_range <- sapply(2:100, function(x) as.Date(x, origin = "2023-01-01"))
+  dat <- data.frame(date = dates, value = rnorm(50))
+  res <- model_out(dat, xcol = "date", xtype = "date")
+
+  m <- stats::loess(value ~ as.numeric(date), data = dat)
+  xdf <- tibble::tibble(date = full_range)
+  expected <- stats::predict(m, xdf)
+
+  expect_true(all(res$y == expected))
+  expect_equal(res$x[1], "2023-01-03")
+  expect_equal(res$x[2], "2023-01-04")
+  expect_equal(res$x[99], "2023-04-11")
+})
diff --git a/tests/testthat/test-read.R b/tests/testthat/test-read.R
index c0b0d27..6dc032d 100644
--- a/tests/testthat/test-read.R
+++ b/tests/testthat/test-read.R
@@ -289,3 +289,34 @@ test_that("error running the model results in a 400", {
   expect_equal(body$errors[1, "detail"],
                "day has insufficient unique values to support 10 knots: reduce k.")
 })
+
+test_that("can get dataset with dates", {
+  dates <- c("2023/15/01", "2023/16/01", "2023/17/01", "2023/18/01", "2023/20/01")
+  dat <- data.frame(biomarker = "ab",
+                    value = 1:5,
+                    day = dates)
+  router <- build_routes(cookie_key)
+  post_request <- local_POST_dataset_request(dat,
+                                             "testdataset",
+                                             cookie = cookie)
+  expect_equal(router$call(post_request)$status, 200)
+  res <- router$call(make_req("GET",
+                              "/dataset/testdataset/trace/ab/",
+                              HTTP_COOKIE = cookie))
+  expect_equal(res$status, 200)
+  body <- jsonlite::fromJSON(res$body)
+  data <- body$data
+  expect_equal(nrow(data), 1)
+  expect_equal(data$name, "all")
+  expect_equal(unlist(data$raw[1, "x"]), c("2023-01-15", "2023-01-16", "2023-01-17", "2023-01-18", "2023-01-20"))
+  expect_equal(unlist(data$raw[1, "y"]), 1:5)
+
+  full_range_dates <- c("2023-01-15", "2023-01-16", "2023-01-17", "2023-01-18", "2023-01-19", "2023-01-20")
+  expect_equal(unlist(data$model[1, "x"]), full_range_dates)
+  parsed <- lubridate::parse_date_time(dates, c("dmy", "mdy", "ymd", "ydm"))
+  suppressWarnings({m <- stats::loess(value ~ as.numeric(day), data = data.frame(day = parsed,
+                                                               value = 1:5))})
+  parsed_full_range <- lubridate::parse_date_time(full_range_dates, c("dmy", "mdy", "ymd", "ydm"))
+  expected <- stats::predict(m, parsed_full_range)
+  expect_equal(unlist(data$model[1, "y"]), expected)
+})
\ No newline at end of file
diff --git a/tests/testthat/test-upload.R b/tests/testthat/test-upload.R
index a338036..d8ec997 100644
--- a/tests/testthat/test-upload.R
+++ b/tests/testthat/test-upload.R
@@ -46,6 +46,23 @@ test_that("uploading dataset with duplicate name returns 400", {
                "testdataset already exists. Please choose a unique name for this dataset.")
 })
 
+test_that("uploading dataset with invalid xcol values returns 400", {
+  router <- build_routes(cookie_key)
+  request <- local_POST_dataset_request(data.frame(biomarker = "ab",
+                                                   day = c("a", "b"),
+                                                   value = 1),
+                                        "testdataset",
+                                        cookie = cookie)
+  res <- router$call(request)
+  expect_equal(res$status, 400)
+
+  res <- router$call(request)
+  body <- jsonlite::fromJSON(res$body)
+  validate_failure_schema(res$body)
+  expect_equal(body$errors[1, "detail"],
+               "Invalid x column values: these should be numbers or dates in a standard format.")
+})
+
 test_that("can upload dataset with different xcol", {
   router <- build_routes(cookie_key)
   request <- local_POST_dataset_request(data.frame(biomarker = "ab",