English to Hebrew Other General / Conversation / Greetings / Letters

e.g. in "two assessments, at least 21 days apart".
I used to translate such sentences as:
"שתי בדיקות, בהפרש 21 ימים לפחות"
Until a certain editor remarked that "הפרש" cannot be applied to time. Ever since this word gives me a headache every time, I can't think of any alternative that doesn't sound bad or is incredibly cumbersome.
I will appreciate any suggestion (I wouldn't even mind hearing you think that editor was wrong and "הפרש" is fine :) )