I'm thinking that this episode basically explained the mystery behind Daniel's crying (4.01 flashback) and his apparent memory issues..
In this episode (The Constant), the final scene shows us that Daniel's journal has been updated by the past version of himself - this was I assume, due to Desmond's time-travelling replanting new events and memories (paths) in his life. With this in mind, it seems logical to assume that
past Daniel waited for the moment until his timeline would catch-up with
future Desmond's timeline (i.e. 2004). Past Faraday knew about the island due to Desmond telling him about it, and probably spent the next 8 years planning and researching to go to the island and Desmond (i.e. Penny), therefore when he saw news of 815 being 'confirmed dead' he probably believed that this was his moment, and he responded with tears of joy..relief?
Of course he then forgot why he was crying..this also points to Desmond's time-travelling which ultimately
screwed up Faraday's memory (much like Desmond's own in during his time-travelling).
By telling Faraday the future (and remember, "you can't change the future"..but you CAN change the past
) it meant that Faraday has also been living in two times (if you will)..not literally like Desmond, but mentally - and consequently he required a "constant"..just like Desmond did.
Hopefully I'll be able to articulate my thoughts on this better soon
