Is it possible to calculate the candle lighting time based on location and date alone?
Assume all you have is a GPS, a watch and a calculator.
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Theoretically yes (assuming candle lighting is just 18 minutes less than sunset), as here's the formula for sunset time based on date, latitude, and longitude; but it's a doozy!
I believe the common custom is indeed calculated based on latitude and longitude alone. However, this is certainly not a unanimous assumption, considering that in addition to latitude and longitude, your own elevation, as well as the elevation of the western horizon, will affect the appearance of sunset. I believe Yeshivat Birkat Moshe, in Maale Adumim, under the auspices of Rav Nachum Rabinovitch, shelita"h, accepts Shabbat well before the sea-level calculations of the popular sunset tables.
See Shabbath 118b:
א"ר יוסי יהא חלקי ממכניסי שבת בטבריא וממוציאי שבת בצפורי
Said Rabbi Yosi: Let my lot be of those who enter the Sabbath in Tiberia (lower elevation = earlier time; see Rashi ad loc) and of those who leave Sabbath in Zippori (higher elevation = later time).