Yesterday I came across some grammatical rules which said “に is used to indicate a specific time or date, but in the case of a relative date, に is not used. I know relative time include terms like yesterday, tomorrow, and today. But can someone give me a more accurate definition of relative date? Thank you~
Absolute times are ones that stay where they are as time moves onwards, or to think of it another way they're the ones that you can circle on a calendar or measure on a clock. So "Tuesday" or "May 25th" or "4 AM" are all absolute times, and the ever-changing now will eventually catch up to them and then pass them.
Relative times are ones measured in relation to when "now" is - so "5 minutes ago", "tomorrow", "next year" are all relative because the specific time they reference will change - "5 minutes ago" might have meant "4 AM" when you said it, but eventually it will mean "noon" or "1:47".
An absolute time or date is a time period that will refer to the same time, regardless of when it is mentioned. These are typically exact times (on the same day in the current timezone unless otherwise stated) or calendar dates. In Japanese these are usually marked with the に particle but this can be omitted in casual conversation. For example:
6月25日 (June 25)
A relative time refers to a different time or day depending on when it is mentioned. In Japanese these can be followed by は, が, or no particle at all. They're often given at the start of a sentence. For example:
一時間後 (an hour later)
一年前 (a year ago)
再来週 (the week after next)
Note that both relative and absolute times can be used with から (beginning) or まで (ending) for time intervals.
ConMan and Tom Kelly have addressed the question you have asked in the subject/title of your post. As for the rather different one in the body, it sounds to me as if your source is simply using "relative time" for times measured in hours or smaller units of time ("three seconds ago," "forty-five minutes from now," "forty-eight hours ago," etc.) and "relative date" for times measured in days or larger units ("the day after tomorrow," "two weeks ago," "eight months from now," "the year after next," etc.").