It's a very good question, don't you think?
Well some friends of mine just invented a formula to calculate Greek time approximately!
GT = T*[1 - T/(HS*60)] + HT*dwf*ff
GT - Greek time[min]
T - Time as the rest of the world understands [min]
HS - Hours of sun [hours/day]
HT - Halara time needed for average Greek native [min/day]
dwf - day of the week corrective factor [0; 1]
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As an example, on a normal Friday in August after lunch/dinner, 5 Greek minutes would actually be...
GT = 5*[1-5/(12,08*60)] + 240*0,9*0,2
GT = 48,17 min
I'd say it's pretty accurate ;)