diff --git a/src/function.jl b/src/function.jl
index de80af6..4b24bb3 100644
--- a/src/function.jl
+++ b/src/function.jl
@@ -76,7 +76,7 @@ function cache(@nospecialize(f), path::AbstractString; overwrite = false)
         if ext == ".bson"
             data = Dict(
                 :version => version,
-                :whenrun => whenrun,
+                :whenrun => string(whenrun),
                 :runtime => runtime,
                 :output => output,
             )
diff --git a/test/items/function.jl b/test/items/function.jl
index 8cd010f..77be053 100644
--- a/test/items/function.jl
+++ b/test/items/function.jl
@@ -54,7 +54,7 @@
             if ext == "bson"
                 data = BSON.load(path)
                 version = data[:version]
-                whenrun = data[:whenrun]
+                whenrun = DateTime(data[:whenrun])
                 runtime = data[:runtime]
             else
                 data = JLD2.load(path)