diff --git a/darwin/datetimepicker.m b/darwin/datetimepicker.m
index fadf94ca..3c09e72a 100644
--- a/darwin/datetimepicker.m
+++ b/darwin/datetimepicker.m
@@ -87,6 +87,8 @@ void uiDateTimePickerTime(uiDateTimePicker *d, struct tm *time)
 	date = [d->dp dateValue];
 	t = (time_t) [date timeIntervalSince1970];
 
+	// Copy time to minimize a race condition
+	// time.h functions use global non-thread-safe data
 	tmbuf = *localtime(&t);
 	memcpy(time, &tmbuf, sizeof(struct tm));
 }
@@ -96,6 +98,7 @@ void uiDateTimePickerSetTime(uiDateTimePicker *d, const struct tm *time)
 	time_t t;
 	struct tm tmbuf;
 
+	// Copy time because mktime() modifies its argument
 	memcpy(&tmbuf, time, sizeof(struct tm));
 	t = mktime(&tmbuf);
 
diff --git a/ui.h b/ui.h
index 765b7733..b5fb9a27 100644
--- a/ui.h
+++ b/ui.h
@@ -251,6 +251,7 @@ _UI_EXTERN uiRadioButtons *uiNewRadioButtons(void);
 struct tm;
 typedef struct uiDateTimePicker uiDateTimePicker;
 #define uiDateTimePicker(this) ((uiDateTimePicker *) (this))
+// TODO document that tm_wday and tm_yday are undefined, and tm_isdst should be -1
 _UI_EXTERN void uiDateTimePickerTime(uiDateTimePicker *d, struct tm *time);
 _UI_EXTERN void uiDateTimePickerSetTime(uiDateTimePicker *d, const struct tm *time);
 _UI_EXTERN void uiDateTimePickerOnChanged(uiDateTimePicker *d, void (*f)(uiDateTimePicker *, void *), void *data);
diff --git a/unix/datetimepicker.c b/unix/datetimepicker.c
index dc2a0328..5f052dca 100644
--- a/unix/datetimepicker.c
+++ b/unix/datetimepicker.c
@@ -576,6 +576,8 @@ void uiDateTimePickerTime(uiDateTimePicker *d, struct tm *time)
 	t = g_date_time_to_unix(dt);
 	g_date_time_unref(dt);
 
+	// Copy time to minimize a race condition
+	// time.h functions use global non-thread-safe data
 	tmbuf = *localtime(&t);
 	memcpy(time, &tmbuf, sizeof(struct tm));
 }
@@ -585,6 +587,7 @@ void uiDateTimePickerSetTime(uiDateTimePicker *d, const struct tm *time)
 	time_t t;
 	struct tm tmbuf;
 
+	// Copy time because mktime() modifies its argument
 	memcpy(&tmbuf, time, sizeof(struct tm));
 	t = mktime(&tmbuf);