When you add more currencies you your Freshsales account you have an option to choose whether you want to manually enter the conversion rates or Freshsales should automatically update these rates. These conversion rates will be used to convert the Deal Value in your account's base currency. The deal value in base currency can be used while generating reports.
If you choose Auto as the Conversion Rate Type, Freshsales uses the exchange rates available through Open Exchange Rates.
Here's how Freshsales picks currency conversion rates:
If today is 25th February 2019,
The previous day's (24th February 2019) closing rates will be synced with Freshsales from the exchange rate sources.
The synced rates will be used for deals created (or closed) from 00:01 UTC onwards.
For deals occurring (created or closed) before 00.01 UTC, Freshsales will use the closing rates from 23rd February 2019.
If Freshsales fails to sync with the exchange rate sources on 25th August 2019, the last available exchange rates will be used.
For deals dated prior to 1999, the closest available exchange rate for 1999 will be used for conversion.