diff --git a/tests/testthat/test-gsSurvCalendar.R b/tests/testthat/test-gsSurvCalendar.R
new file mode 100644
index 00000000..d109caae
--- /dev/null
+++ b/tests/testthat/test-gsSurvCalendar.R
@@ -0,0 +1,133 @@
+test_that("gsSurvCalendar basic functionality works", {
+  # Test default parameters
+  x <- gsSurvCalendar()
+  expect_s3_class(x, c("gsSurv", "gsDesign"))
+  expect_equal(x$k, 3) # Default number of analyses
+  expect_equal(x$test.type, 4) # Default test type
+
+  # Test calendar time input validation
+  expect_error(
+    gsSurvCalendar(calendarTime = c(24, 12, 36)),
+    "calendarTime must be an increasing vector"
+  )
+
+  # Test spending type options
+  x_info <- gsSurvCalendar(spending = "information")
+  x_cal <- gsSurvCalendar(spending = "calendar")
+  expect_false(identical(x_info$n.I, x_cal$n.I))
+
+  # Test minimum follow-up validation
+  expect_error(
+    gsSurvCalendar(calendarTime = c(12, 24), minfup = 25),
+    "Minimum follow-up greater than study duration."
+  )
+})
+
+test_that("gsSurvCalendar matches gsSurv under specific conditions", {
+  # Create comparable designs
+  cal_design <- gsSurvCalendar(
+    test.type = 4,
+    calendarTime = c(12, 24, 36),
+    spending = "information",
+    minfup = 6
+  )
+
+  # Get event counts for timing
+  nevents <- sapply(cal_design$T, function(x) {
+    xx <- nEventsIA(tIA = x, x = cal_design, simple = FALSE)
+    sum(xx$eDC) + sum(xx$eDE)
+  })
+  timing <- nevents / max(nevents)
+
+  # Create matching gsSurv design
+  surv_design <- gsSurv(
+    k = 3,
+    test.type = 4,
+    timing = timing,
+    T = max(cal_design$T),
+    minfup = 6
+  )
+
+  # Compare key outputs
+  expect_equal(cal_design$n.I, surv_design$n.I, tolerance = 1e-4)
+  expect_equal(cal_design$lower$bound, surv_design$lower$bound, tolerance = 1e-4)
+  expect_equal(cal_design$upper$bound, surv_design$upper$bound, tolerance = 1e-4)
+})
+
+test_that("gsSurvCalendar handles different spending functions", {
+  # Test with O'Brien-Fleming spending
+  x_of <- gsSurvCalendar(
+    sfu = gsDesign::sfLDOF,
+    sfl = gsDesign::sfLDOF,
+    calendarTime = c(12, 24, 36),
+    spending = "information"
+  )
+  expect_s3_class(x_of, c("gsSurv", "gsDesign"))
+
+  # Test with Pocock spending
+  x_pocock <- gsSurvCalendar(
+    sfu = gsDesign::sfLDPocock,
+    sfl = gsDesign::sfLDPocock,
+    calendarTime = c(12, 24, 36),
+    spending = "information"
+  )
+  # Do Pocock spending with calendar time
+  x_pocock_cal <- gsSurvCalendar(
+    sfu = gsDesign::sfLDPocock,
+    sfl = gsDesign::sfLDPocock,
+    calendarTime = c(12, 24, 36),
+    spending = "calendar"
+  )
+  expect_s3_class(x_pocock_cal, c("gsSurv", "gsDesign"))
+
+  # Verify different spending functions produce different bounds
+  expect_false(identical(x_of$upper$bound, x_pocock$upper$bound))
+  expect_false(identical(x_pocock$upper$bound, x_pocock_cal$upper$bound))
+  # Verify cumulative spending matches spending function for information spending
+  expect_equal(
+    cumsum(x_pocock$upper$spend),
+    gsDesign::sfLDPocock(
+      alpha = x_pocock$alpha,
+      t = x_pocock$upper$sTime
+    )$spend,
+    tolerance = 1e-4
+  )
+  # Verify cumulative spending matches spending function for calendar time
+  expect_equal(
+    cumsum(x_pocock_cal$upper$spend),
+    gsDesign::sfLDPocock(
+      alpha = x_pocock_cal$alpha,
+      t = x_pocock_cal$upper$sTime
+    )$spend,
+    tolerance = 1e-4
+  )
+  # Verify spending time is proportion of full T
+  expect_equal(x_pocock_cal$upper$sTime, x_pocock_cal$T / max(x_pocock_cal$T))
+})
+
+test_that("gsSurvCalendar output structure is correct", {
+  x <- gsSurvCalendar()
+
+  # Check essential components
+  expected_names <- c(
+    "k", "test.type", "alpha", "beta", "astar",
+    "delta", "n.fix", "timing", "tol", "r",
+    "n.I", "maxn.IPlan", "nFixSurv", "nSurv", "endpoint",
+    "delta1", "delta0", "overrun", "usTime", "lsTime",
+    "upper", "lower", "theta", "falseposnb", "en",
+    "hr", "hr0", "R", "minfup", "gamma",
+    "eDC", "eDE", "eNC", "eNE", "ratio",
+    "lambdaC", "etaC", "etaE", "variable", "T"
+  )
+
+  expect_true(all(expected_names %in% names(x)))
+
+  # Check dimensions of key components
+  expect_equal(length(x$n.I), x$k)
+  expect_equal(length(x$upper$bound), x$k)
+  expect_equal(length(x$lower$bound), x$k)
+
+  # Check class attributes
+  expect_true(inherits(x$upper, "spendfn"))
+  expect_true(inherits(x$lower, "spendfn"))
+})