If you only need the time, you may try this:
1. Hook Ham_Jump and cache get_gametime value.
2. Enable a touch forward between the player and the worldspawn.
3. In the Ham forward above check if the player is on ground( FL_ONGROUND ), then use get_gametime again.
4. Disable the Ham_Touch forward.
5. The time while the player was in the air is the difference between them.
I don't quite understood what you want, but you may also check m_flFallVelocity offset.