Similar to this issue, it seems like the timestamps from the replay file do not match the ones in the Riot API.
These are the timestamps for some of the champion deaths for this game (API key required) from each data source. There's clearly some kind of relationship, but I don't know precisely what it is.
| Replay |
Riot API |
| 74715 |
92622 |
| 365176 |
234778 |
| 457867 |
278090 |
| 753802 |
415512 |
| 948020 |
509836 |
| 1039259 |
552867 |
| 1045446 |
555405 |
| 3040823 |
1551262 |
The replay timestamps are from the "SetDeathTimer" in the replay chunks.