Whatsapp time stamp on actual message received changed before my eyes

Can someone help me understand how I woke in the morning and looked at my conversation with my partner from the night before , and the last message received said 11.47pm , then I looked again ten minutes later to see if he was up and online I noticed the last message received had changed to a different time a few minutes difference ? And he had not been online since the night before . We both have automatic time , does this insinuate my partner changed his time and date at some point while being in whatsapp during the early hours for some reason to hide him being online to me ?. I just tried it on my own whatsapp , I changed time and date to last night and logged into whatsapp and hey presto, the last few messages from this morning have all had their times changed to around 4 am . I'm now thinking my partner is sneaking around, why else would the time on a message change without him being online or without him dabbling with his time settings ?