You can convert all your other currencies into USD using the exact day when you created those expenses in that currency. Xe.com is a good reference for this.
One thing to be careful about... the currency exchange and the exchange rate are 2 different things. You need to calculate that currency exchange cost as well and most online currency converter will not show you this info.
Whenever you convert money into a different currency, part of this conversion will include a certain rate for this operation and this can vary depending on your financial institution or payment method. This is a little extra fee you pay for getting that currency conversion service.
Usually, it's safe to add at least 2-5% (usually 2.5% in most banks/credit cards) to your conversion to make sure you're not actually paying for it when you'll get your expenses refunded. This is some kind of hidden markup, and when you convert your money, it might not appear as a separate fee on the cost of conversion and will be shown as the exchange rate.
You mentioned in a comment that the currency value changes all day; indeed you're right, you can't be 100% precise with this. With the currency conversion rate, that's where you can add an extra 0.1-0.5% (or what you want) to your conversion to compensate for the fluctuations that could get you in the negative. In a way, you also need to get a compensation for the administration of that currency exchange and the extra paperwork it gives you; that's why it's fair to add a little extra percentage just for this.
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