ISAAC,

the Subroutine "CalculateLinearFromDate" isn't really that complex, is it?

with your variables beeing:

Date =23
Month=03
Year=05
Hour =21
Mins =30
Seconds=30

all you need to do is:
rename your VAR "Date" to "DAY"
(or create an alias)
and declare the following variables in addition:

CounterA VAR Byte
TempA VAR WORD
TempB VAR WORD
DAYS WAR WORD

once you have read the DS1307 just

GOSUB CalculateLinearFromDate

and you will find the result in "DAYS"