This might be more a typography related question than actual language question, but I didn’t know where else to turn.
First of, I am perfectly aware that it’s generally considered bad form—regardless of style guide—to start a sentence with digits. However, therein lies the problem; it’s a date
The style guide in question specifies that in this instance, one should use digits. So what does one do in such an instance, where you have to start a sentence—for whatever reason—with a digit.
The options, as I see them, are;
Capitalise the date, lead with a titling figure and keep the rest in text figures.
Use only text figures, and breaking the rule about starting a sentence in the wrong case.
Use only titling figures and make your body text look silly.
Toss the style guide and avoid the problem. (Albeit a perfectly valid option, I don’t want to do that, mostly because I really want to know how one should capitalise digits.)
The date format in question, using today’s date: 21 October 2013.
For the sake of argument, let’s exclude the “prefixing the date with ‘the’,” option, shall we?
Edit: Examples illustrated as per request, in order mentioned.