It is set in CIV5GameSpeeds.xml file, For each speed you define the number of turns and months per turn to cover the 6000 years of the game. Below is the section for the standard game. You cannot start at an earlier date, but you can stretch the number of turns, in theory up to 72,000 turns in one month increments. This same file also has the variables to set the costs. Set <ResearchPercent> to a number greater tha 100 to slow the standard rersearch percentages. The game speeds do this, but also increase all the other costs. So Marathon has most costs set to 300.
<Row>
<GameSpeedType>GAMESPEED_STANDARD</GameSpeedType>
<MonthIncrement>480</MonthIncrement>
<TurnsPerIncrement>75</TurnsPerIncrement>
</Row>
- <Row>
<GameSpeedType>GAMESPEED_STANDARD</GameSpeedType>
<MonthIncrement>300</MonthIncrement>
<TurnsPerIncrement>60</TurnsPerIncrement>
</Row>
- <Row>
<GameSpeedType>GAMESPEED_STANDARD</GameSpeedType>
<MonthIncrement>240</MonthIncrement>
<TurnsPerIncrement>25</TurnsPerIncrement>
</Row>
- <Row>
<GameSpeedType>GAMESPEED_STANDARD</GameSpeedType>
<MonthIncrement>120</MonthIncrement>
<TurnsPerIncrement>50</TurnsPerIncrement>
</Row>
- <Row>
<GameSpeedType>GAMESPEED_STANDARD</GameSpeedType>
<MonthIncrement>60</MonthIncrement>
<TurnsPerIncrement>60</TurnsPerIncrement>
</Row>
- <Row>
<GameSpeedType>GAMESPEED_STANDARD</GameSpeedType>
<MonthIncrement>24</MonthIncrement>
<TurnsPerIncrement>50</TurnsPerIncrement>
</Row>
- <Row>
<GameSpeedType>GAMESPEED_STANDARD</GameSpeedType>
<MonthIncrement>12</MonthIncrement>
<TurnsPerIncrement>120</TurnsPerIncrement>
</Row>
- <Row>
<GameSpeedType>GAMESPEED_STANDARD</GameSpeedType>
<MonthIncrement>6</MonthIncrement>
<TurnsPerIncrement>60</TurnsPerIncrement>
</Row>