It's using bytewise string-manipulation functions and apparently assuming every character in the string is 2 bytes. But multiples of 1000 include U+0027 in the string generated to that point, which is only one byte.
Wikitext such as {{#time:xhxjj xjx xhxjY|4239-10-12}} will produce such a call, which may result in oddly truncated or empty output from the parser.