From that angle, there are several joints that show clear signs of insufficient wetting of the pad. The solder has balled up around the pin but has not formed a solid electrical bond with the surrounding solder pad. Joints like these are not reliable and tend to get worse over time.
Retouch these (and any others where the solder has balled up around the pin). Put the tip of the hot iron on the solder pad and heat until the solder flows from the pin down onto the pad.
Hi everybody. I wanted to let you know that I was having precisely the same issue.
I was able to correct the issue by re-verifying the installation of components.
The ultimate issue was the diode that is installed by the battery (D4, I believe) was installed backwards. This produced the effect of the clock working for a while, then shutting itself off. Resetting the power allowed it to turn back on - but the clock was reset.
So - if you have the same issue, check your diode by the battery backup and make sure it is installed in the correct orientation.