[FIX] fix datetime addition

[holg]

The chrono::Duration object doesn't have += operation defined. So instead of using +=, the addition should be done explicitly. It worked before, but this made it not being compiled, so i fixed it.

[AaronErhardt]

It seems like support for += was added in chrono 0.4.34, but plotters only lists v0.4.20 as baseline requirement. So instead of undoing the recent commits that fixed some clippy lints, I suggest to update the baseline for chrono to 0.4.34.

[holg]

Instead of doing this fix the update the baseline for chrono to 0.4.34.