Historical clue formulations

• Surreptitious bit of communication — 2021-03-21
• Indicator of interest on Match.com — 2020-01-18
• Instant — 2019-01-09
• Part of ;-) — 2015-08-29
• [I'm kidding!] — 2010-11-11
• Sleep unit? — 2010-11-07
• Indication that one is just teasing — 2008-10-03
• Instant — 2006-01-19
• [Just kidding!] — 2001-09-30
• Secret sign — 1999-11-02
• Flirtatious gesture — 1999-04-29
• Split second — 1998-10-01
• [Get the joke?] — 1997-09-18
• Subtle signal — 1996-12-07