Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
The offset you gave to the date builtin is not a valid offset. Here is the shape it wants. When you turn a number of milliseconds into ISO text, you can say, with b, which offset to write it in. The offset is either a capital Z, for UTC, or a plus or minus sign followed by hours and minutes with a colon, like plus zero three colon zero zero. The hours go from zero to twenty-three, and the minutes from zero to fifty-nine. Something like a plus twenty-five hours, or a name of a city, is not a valid offset, and rush will not try to figure out what you meant. There is also a second case: when you read a text into an instant, you do not pass b at all, because the text already carries its own offset, and two offsets would compete. The first slot shows what you gave, and the second says what is wrong. Write the offset in the exact form, or remove b when you are reading text, and the call will go through.
Text from the Rush engine's error catalogue.