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diff --git a/tests/test/datetime-util.scm b/tests/test/datetime-util.scm
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+;;; Commentary:
+;; Tests timespan overlaps and month-streams.
+;; Separate from tests/datetime.scm since
+;; (datetime util) originally was its own module.
+;;; Code:
+
+(define-module (test datetime-util)
+  :use-module (srfi srfi-64)
+  :use-module (srfi srfi-88)
+  :use-module ((datetime)
+               :select (date time
+                             datetime
+                             month-stream
+                             in-date-range?
+                             timespan-overlaps?))
+  :use-module ((srfi srfi-41)
+               :select (stream->list stream-take)))
+
+(test-assert
+  "jan->dec"
+  (stream->list
+    (stream-take
+      11
+      (month-stream
+        #2020-01-01))))
+
+(test-assert
+  "dec->jan"
+  (stream->list
+    (stream-take
+      2
+      (month-stream
+        #2020-12-01))))
+
+(test-assert
+  "dec->feb"
+  (stream->list
+    (stream-take
+      3
+      (month-stream
+        #2020-12-01))))
+
+(test-assert
+  "20 months"
+  (stream->list
+    (stream-take
+      20
+      (month-stream
+        #2020-01-01))))
+
+(test-equal
+  "Correct months"
+  (list #2020-02-01
+        #2020-03-01
+        #2020-04-01
+        #2020-05-01
+        #2020-06-01
+        #2020-07-01
+        #2020-08-01
+        #2020-09-01
+        #2020-10-01
+        #2020-11-01
+        #2020-12-01
+        #2021-01-01)
+  (stream->list
+    (stream-take
+      12
+      (month-stream
+        #2020-02-01))))
+
+(test-assert
+  "in-date-range?"
+  (not ((in-date-range?
+          #2020-01-01
+          #2020-02-29)
+        #2018-02-02)))
+
+(test-assert
+  "A"
+  (timespan-overlaps?
+    #2020-01-01
+    #2020-01-10
+    #2020-01-05
+    #2020-01-15))
+
+(test-assert
+  "A, shared start"
+  (timespan-overlaps?
+    #2020-01-01
+    #2020-01-10
+    #2020-01-01
+    #2020-01-15))
+
+(test-assert
+  "A, tangential"
+  (not (timespan-overlaps?
+         #2020-01-01T00:00:00
+         #2020-01-10T00:00:00
+         #2020-01-10T00:00:00
+         #2020-01-30T00:00:00)))
+
+(test-assert
+  "s1 instant"
+  (timespan-overlaps?
+    #2020-01-15T10:00:00
+    #2020-01-15T10:00:00
+    #2020-01-10T00:00:00
+    #2020-01-30T00:00:00))
+
+(test-assert
+  "s2 instant"
+  (timespan-overlaps?
+    #2020-01-10T00:00:00
+    #2020-01-30T00:00:00
+    #2020-01-15T10:00:00
+    #2020-01-15T10:00:00))
+
+(test-assert
+  "s1 instant, shared start with s2"
+  (timespan-overlaps?
+    #2020-01-15T10:00:00
+    #2020-01-15T10:00:00
+    #2020-01-15T10:00:00
+    #2020-01-30T00:00:00))
+
+(test-assert
+  "s1 instant, shared end with s2"
+  (not (timespan-overlaps?
+         #2020-01-15T10:00:00
+         #2020-01-15T10:00:00
+         #2020-01-10T00:00:00
+         #2020-01-15T10:00:00)))
+
+(test-assert
+  "s2 instant, shared start with s1"
+  (timespan-overlaps?
+    #2020-01-15T10:00:00
+    #2020-01-30T00:00:00
+    #2020-01-15T10:00:00
+    #2020-01-15T10:00:00))
+
+(test-assert
+  "s2 instant, shared end with s1"
+  (not (timespan-overlaps?
+         #2020-01-10T00:00:00
+         #2020-01-15T10:00:00
+         #2020-01-15T10:00:00
+         #2020-01-15T10:00:00)))
+
+(test-assert
+  "both instant"
+  (not (timespan-overlaps?
+         #2020-01-15T10:00:00
+         #2020-01-15T10:00:00
+         #2020-01-15T10:00:00
+         #2020-01-15T10:00:00)))
+
+(test-assert
+  "tangential whole day"
+  (not (timespan-overlaps?
+         #2020-01-01
+         #2020-01-02
+         #2020-01-02
+         #2020-01-03)))
+
+(test-assert
+  "B"
+  (timespan-overlaps?
+    #2020-01-05
+    #2020-01-15
+    #2020-01-01
+    #2020-01-10))
+
+(test-assert
+  "E"
+  (timespan-overlaps?
+    #2020-01-01
+    #2020-01-10
+    #2020-01-01
+    #2020-01-10))
+
+