ADT shows error in Strings.xml
Or, alternatively, you can modify your % format specifiers to use argument indices, like so:
<string name="date_format">%1$d:%2$d %3$s</string>
More info on the Formatter class in the Android Reference Docs.
Try doing the following
<string name="date_format" formatted="false">%d:%d %s</string>
The detailed explanation can be found in the answer to this question: Android XML Percent Symbol