Since we live in western Washington and the weather dramatically affects the time of day it gets light (regardless of sunrise/sunset times), we don't pay much attention to daytime as ECS is programmed.  We instead opted to use light sensors and use them to control an item called 'Daylight'.  First we received a couple of Sundowners as part of a promotion from x10.com.   To make sure we detect the transition at the correct time, we use both of the Sundowners; one set to maximum sensitivity and the other set to minimum sensitivity.  Each Sundowner has its own address (they could actually share one but we wanted to track which sensor triggered at what time).  The event to track usage looks like this:

If PLC-Button G Is None                ->This section just allows us to ignore the rest
And PLC-Command G Is Not Dim           ->of the event if there hasn't been anything
And PLC-Command G Is Not Bright        ->received for this house code (G).
Then Event-Exit Set True
Do Click Set 1
;----------------------------------------------------------------
BEGINIF PLC-Button G Is 1/On           ->The unit code for the first sensor is 1 and
Then Text-A T1<-T1 Text, G1            ->section is to respond to an on indication from
Then Text-A Set <Space>                ->the Sundowner.  Note the Sundowner is designed
Then Temp State 4 Set Off              ->to operate a light if it becomes dark so the
Then Text-A T1/A<-Stt Temp State 4     ->indication may seem backwards.  We actually
Then Text-A Set Log/Speak              ->invert the indication prior to entering it into
Then Sens, Light, 1 Set False          ->the log.  In the next section, we will explain
ENDIF                                  ->what is going on.
;----------------------------------------------------------------
BEGINIF PLC-Button G Is 1/Off          ->If G1/Off is sent
Then Text-A T1<-T1 Text, G1            ->Copy the text contents of Text, G1 to Text-A
Then Text-A Set <Space>                ->Append a space to the end of Text-A
Then Temp State 4 Set On               ->Change the state of our temp state holder to on
Then Text-A T1/A<-Stt Temp State 4     ->Copy the state of the state holder to Text-A
Then Text-A Set Log/Speak              ->Speak and log the contents of Text-A
Then Sens, Light, 1 Set True           ->Set the item related to this sensor to True
ENDIF                                  ->Close this BEGINIF loop
;----------------------------------------------------------------
BEGINIF PLC-Button G Is 2/On
Then Text-A T1<-T1 Text, G2
Then Text-A Set <Space>
Then Temp State 4 Set Off
Then Text-A T1/A<-Stt Temp State 4
Then Text-A Set Log/Speak
Then Sens, Light, 2 Set False
ENDIF
;----------------------------------------------------------------
BEGINIF PLC-Button G Is 2/Off
Then Text-A T1<-T1 Text, G2
Then Text-A Set <Space>
Then Temp State 4 Set On
Then Text-A T1/A<-Stt Temp State 4
Then Text-A Set Log/Speak
Then Sens, Light, 2 Set True
ENDIF

The Daylight event is even simpler:

If Sens, Light, 1 Is Now False
Then Daylight Set False
If Sens, Light, 2 Is Now False
Then Daylight Set False
If Sens, Light, 1 Is Now True
Then Daylight Set True
If Sens, Light, 2 Is Now True
Then Daylight Set True

Now any time we need to know if it is light or dark outside, we check the status of the item 'Daylight'.  This way we don't have to know what our long/lat is or even what time zone we are in to guess whether it is daylight or not.

Something worthy of not is all the sensor items should be set to be backed up so if the PC has to be rebooted, it will remember the status of Daylight.  The sensors must be placed where only natural light will hit them or you will get all kinds of unreliable, undesirable, and unexpected results.