Why does the last seen timestamp on Facebook Messenger change?

[Anonymous]

Mates do you know why this just happens? I sent a message to a friend on Dec 17 and I'm so sure she read it that day cause I received the seen icon but now it is written 'seen on Dec 18'
Do you know why this happens really?

[Errhh]

Midnight? Could've been night time and you took it as the day before when it was already the next day? But who cares? Why people care about insignificant things such as this.. What difference does it matter... Weird!!!

[Kachatterjee]

She was online, connected to internet on Dec 17. However she opened up the messenger on Dec 18 and read it. Seen icon actually was designed when the text is deliveres hough not seen.